WA/91/4 Applied Geological Mapping in the Wrexham Area: Geology and Land- Use Planning

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TECHNICAL REPORT WA/91/4 Applied geological mapping in the Wrexham area: geology and land- use planning

B A Hains

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BRITISH GEOLOGICAL SURVEY

TECHNICAL REPORT WA/91/4 Onshore Geology Series

Applied geological mapping in the Wrexham area: geology and land-use planning

B A Hains

Contributors M G Culshaw Engineering geology R A Monkhouse Hydrogeology Cover illustration The A483 from Chester to Wrexham (Gresford bypass)has been built through the northern part of the gorge of the River Alyn, partly through on soft alluvial sediments, and partly on waste from the now-disused Gresford Colliery.

This study was commissioned by the Department of the Environ- ment, but the views expressed in it are not necessarily those of the Department

Maps and diagrams in thisbook use topography basedon Ordnance Survey mapping

Geographical index UK, Wales, Clwyd

Subject index Land-use planning, thematic maps, resources, aggregate, mining, engineering geology, ground stability, hydrogeology

Bibliographic reference Hains, B A. 1991.Applied geological mapping in the Wrex- ham area: geology and land-use planning. British Geological Survey Technical Report WAl9114.

The full range of Survey publications is available through the Keyworth, Nottingham NG12 5GG Sales Desks at Keyworth and at Murchison House, Edinburgh, e Plumtree (06077) 6111 Telex378173 BGSKEY G and in the BGS London Information Office in the Natural Fax 06077-6602 History Museum Earth Galleries. The adjacent bookshop stocks the more popular books for sale over the counter. Most BGS Murchison House, West Mains Road, Edinburgh EH93LA books and reports are listed in HMSO’s Sectional List 45, and can be bought from HMSO and through HMSO agents and e 031-6671000Telex727343 SEISED G retailers. Maps are listed in the BGS Map Catalogue and the Fax 031-668 2683 Ordnance Survey’s Trade Catalogue, and canbe bought from Ordnance Survey agents as well as from BGS. London Information Office at the Natural History Museum Earth Galleries, Exhibition Road, South Kensington, London The British Geological Survey carries out the geological survey SW7 2DE of Great Britain and Northern Ireland (the latter as an agency e 071-5894090 Fax 071-584 8270 service for the government of Northern Ireland), and ofthe 071-938 9056/57 surrounding continental shelfi as well as its basic research projects. It also undertakes programmes of British technical aid 19 Grange Terrace, Edinburgh EH9 2LF in geology in developing countries as arranged by the Overseas 031-667 727343Telex1000 SEISED G Development Administration. St Just, 30 Pennsylvania Road, Exeter EX4 6BX The British Geological Survey is a component body of the Exeter(0392) 78312 Fax 0392437505 Natural Environment Research Council. e Bryn Eithyn Hall, Llanfarian, Aberystwyth, Dyfed SY23 4BY e Aberystwyth(0970) 611038 Fax 0970-624822

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Printed in England for the British Geological Survey by Baker Brothers Litho Limited, Pontefract, West Yorkshire PREFACE

This account describes the applied geology of the Wrexham area, encompassing 215 sq km. It covers theOrdnance Survey 1:lO 000 Sheets SJ 24 NE; SJ 25 NE,SE; SJ 34 NW,NE; SJ 35 NW, SW, SE; and partof SJ 35 NE.

The northern partof the district was first geologically surveyed at the 1:10 560 scale between 1879 and 1885; the remaining area was completed, and the northern part partially revised, from1910 to 1913. Publications on economic aspects of the district included a memoir on the metalliferous mining and mineral resourcesof the region (Smith,1921) and two reportsgiving assessments of the sand and gravel reserves (Dunkley,1981; Ball, 1982).

The presentstudy was commissioned by the Departmentof the Environment onbehalf of the Welsh Office. Funding was provided jointly by the Department of the Environment and the British Geological Survey.Its objectives wereto produce updatedgeological maps at1:lO 000 scale, and, together with other available surface and subsurface data, to present thegeological information relevant to land-use planning, development and redevelopment in the form of a set of maps at 1:25 000 scale with accompanying reports. A major aim of the study was to develop computer techniques so that the geological and thematic mapscould all be computer generated on demand and in colour (Loudon et al., 1991) if required. Particular emphasis has been placed on drift deposits, undergroundworkings and mineral resources. Synthesesof the engineering propertiesof the bedrock and drift (superficial) deposits of the area were undertaken (Waine et al., 1990). Limited testing of Coal Measure mudstones to determine their swelling characteristics has been carried out (Entwisle, 1989). A summary of the hydrogeology has been providedby Mr RA Monk- house. Detailed resurvey at 1: 10000 scale, by Drs J R Davies and D Wilson, was limited to the Hope Mountain area;a desk studyof the remainderof the area,including partial photogrammetric interpretation, andlimited field checking was carried out by Drs R Addison (Leeswood Coalfield), B A Hains and TI Williamson (Sheet SJ 35 NE). DrN J Riley provided palaeontological support for the Hope Mountain area. DDr J Evans provided geophysical information pertaining to geological structures in the eastern partof the area. Datacollection and computer coding of borehole logs was carried out by Mr J A Thorburn.The Project Leader was Dr BA Hains. Dr R A B Bazley, Regional Geologist for Wales, was the Nominated Officer for the British Geological Survey, and Mr D B Courtier, Welsh Office, was the Nominated Officer for the Secretary of State.

The ready cooperation of landowners and tenants, quarry companies, local authorities, govern- mental bodies andother holders ofdata during this surveyis gratefully acknowledged.In particular we thank Clwyd County Council, Alyn and Deeside District Council, Wrexham Maelor Borough Council, British Coal, the Welsh Development Agency, Astbury Quarries Ltd, Dennis Ruabon Ltd, Alfred McAlpine Quarry Products Ltd, Tarmac Roadstone Ltd (North West) and Welsh Aggregates Ltd. We also acknowledge many geological and geotechnical consultants.

V CONTENTS

ExecutiveSummary 1 Annex B Data sources 81

Introduction4 C Annex 84Geotechnical database Objectives4 The use of this reportand limitationsits 4 Annex database 88 Presentation of results 6 AnnexE Glossary 89 Geographical and planning background 7 Geographical background 7 Planning background 11 FIGURES

Geological background (Thematic 1 Sketch-map of the study area showing its Maps 1-4) 16 regionalsetting 8 Introduction16 2 Sketch-map showing the component Solid geology 18 1: 10 000sheets of the study area and the Drift geology 20 outlines of the earlier County Sheet Grid 9 Rockhead elevation 24 3 Topography and drainage of the study Drift thickness 27 area 10

Mining and mineral resources (Thematic 4Planning factors 13 Maps 5-7) 29 5 Agriculturalland classification 15

Mining activities - CoaVMetalliferous 29 6Generalised geological sequence 17 Bedrock resources - except 7 Solid geology 19 CoaVMetalliferous 37 8 Drift geology 21 Sand and gravel resources 41 9Rockhead elevation 25

Slope steepness 45 11 Drift thickness 28 12 Mining activities 30 Hydrogeology (Figure 18) 52 13 Correlation of the Bettisfield Formation (Productive Coal Measures) 32 Geological factors for considerationin land-use 14 Bedrock resources 36 planning (Thematic Map 10, Figure 19) 56 15 Sand and gravel resources 42 Mineral resource potential 57 Physical and chemical constraints to 16 Engineering geology -46solid development61 17 Engineering geology -48drift 18 Hydrogeology 52 References 66 19 Mineral resource potential 58 20 Geological factors for consideration in Annex A Summary of geology 70 land-use planning 59

vi TABLES

‘Correlation’ of Coal Seam nomenclature for theLeeswood and Wrexham areasof the North Wales Coalfield 34 Summary of mechanical and chemical property data from limestones 38

Engineering geology units- Solid geology 47

Engineering geology units- Drift (superficial) deposits 49 Slope steepness classes and examplesof land use limitations 51 Licensed abstraction from shafts, bore holes and springs 53 Landfill waste disposal sites54 Chemical analyses of water sources 55

vii

EXECUTIVE SUMMARY

Introduction

The study area covers the northern part of the ‘Denbighshire’ Coalfield and its environs. It is centred on Wrexham, thelargest town in North Wales. This area has beenundergoing a transfor- mation with the decline of coal mining and associated heavy industries and their replacementby an expanding service sector and new light and high-technology industries. Two other significant parts of the local economy, agriculture and the mineral extraction industry, continue toplay an important de.

The decline of the coal industry hasleft a legacy of old workings, abandoned shafts and adits, and spoil heaps. Additionally there are many disused and extant workingsfor sand and gravel, limestone and brick-clay. This history of exploitation of mineral resources has given rise to variable ground conditions which can pose problems for development. Consequently there is a need for detailed up-to-date geological information to enableplanning policies to beapplied inthe most cost-effective manner.

This study has examined and updated the existing geological and other databases, using newly developed computing techniques where applicable. The results are presented in a form intended to aid planners, engineers andgeologists. Two setsof maps and four reportshave been prepared: 1 The main report, on geology and land-use planning, with a set of ten thematic maps at the 1:25 000 scale. 2 A set of nine geohgical maps at the 1:10 000 scale. 3 Three other reports (published separately) on computing techniques (Loudon et al., 1991), engineering geology (Waine et al., 1990) and swelling characteristics of mudrocks (Entwisle, 1989).

The study was funded jointlyby the Departmentof the Environment (onbehalf of the Welsh Office) andthe BritishGeological Survey (BGS). The work was carried out by BGS staff atthe Aberystwyth, Edinburgh, Keyworth and Wallingfordoffices.

Objectives

The main objectivesof the study were: 1 To produce a new set of geological maps at 1:lO 000 scale of the entire area. 2 To produce a set of thematic element maps (mainly at 1:25 000 scale) and to collate various geotechnical andother available informationin reports intended foruse principally by planners and developers. 3 To develop computer techniques to enable the 1:lO 000 geological maps, the thematic maps, graphic logs of boreholes and other information to be produced on demandin colour and where possible.

1 Geology

The area is dominated by sedimentary rocks of Carboniferous age, which lie unconformably on Ordovician rocks, andare themselves overlain unconformablyby Permo-Triassic rocks (Figure 6). The Carboniferous sequence has,its at base, up to225 m of limestone with subordinate mudstones and sandstones. Thisis overlain by up to420 m of Millstone Grit sandstones and mudstones. These are in turn succeeded by up to 520 m of Productive Coal Measures characterised by a cyclic repetition of mudstone, siltstone, sandstone, seatearth and coal. The Productive Coal Measures are succeeded by up to about 900 m of ‘Red Measures’ (Upper Coal Measures). These also commonly show acyclic repetition of strata, butcoal seamsare poorly developed.The succeeding Permo-Triassic sandstones (up to540 m) areentirely covered by thick drift (superficial) deposits, of relatively recent glacial or post-glacial origin, as is most of the outcrop of the Productive Coal Measures and ‘RedMeasures’.

The strata have a general gentle dip towards the east, and in broad terms this is shown in the topography of the district. The high ground in the west reflects the outcropof the massive limestones of the Carboniferous Limestone and the basal sandstoneof the Millstone Grit. Eastwards thereis a gradual slope towards the broadvalley of the River Dee and theCheshire-Shropshire plain; this slope is underlain by the softer, more easily eroded, and largely drift-covered mudstones of the various divisions of the Coal Measures and,finally, the sandstonesof the Permo-Triassic.

Summary of geological factors related to land-use planning

Mineral resources Sand andgravel is the most important mineral resourcein the study area. Itcovers awide area, especially around and to the northof Wrexham. Potential resource areas are delimited on Map 7, indicating where future developmentcould give rise to sterilisation of deposits. Limestone, of proven aggregate qualityand purity,is present around Minera. Resourceswithin the study area arelimited. Resources of brick-clay are limited by thick drift overburden, and sandstone, while extensive, is not considered to beof major importance (Map6). Further deep mining for coal would not appear to beeconomically viable; however, despite extensive shallow mining in the past, potential for opencastextraction still exists within the area of the Productive Coal Measures outcrop where the drift thickness is less than about 30 m (Maps 5 and 10). Most of the solid rock aquifers, apart from the Carboniferous Limestone, area large to extent protected from pollutionby a thick,relatively impermeable coverof drift; water supplies taken from the drift deposits are moreeasily contaminated (Figure 18, Maps 2 and 4).

Constraints to development 5 Much of the district has been mined for coal in the past. Shallow mining, restricted to those areas of the Productive Coal Measuresoutcrop with less than about30 m of drift overburden, may still be a cause of instability or soft ground conditions. Deeper workings, in the eastern

2 part of the coalfield, are unlikely to cause problems. Over1300 shafts and adits are known to exist; others undoubtedlyremain unlocated (Maps 5 and 10). Extensive mining for lead andzinc has taken place south and west of Minera. The extent of many of the workings is not known, and there are some200 known shafts and adits. Thesemay remain open at depth evenif blocked at thesurface (Map 5). In general,solid rock, till and glacial sand and gravel provide reasonable foundation conditions below the weathered zone.Alluvial deposits andpeat in river valleys and kettle holes, and peat on the westernhill slopes haverelatively weak geotechnical parameters (Maps8 and 9). Made groundis extensive andof variable composition. The infill of worked opencast sites and recently restored sand and gravel pitsis usually well documented; earlierpits and quarriesmay have fill of unknown type and cangive rise to foundation problems (Maps2 and 10). Old coalworkings can actas areas for methane accumulation, andthis gas can also accumulate in old, badly designed,landfill sites. Spontaneous combustionmay occur under certain conditions if colliery waste is disturbed (Maps 5 and 10, Figure 18). 10 Steep slopes, especially in driftdeposits or in the mudstones of the Millstone Grit and Productive Coal Measures may prove unstable, and several landslipped areas have been recorded. Further slips may occur if such slopes are disturbed either by human activity or undercutting by river erosion (Maps1 , 2 and 9). 11 Other potential constraints include environmental factors (e.g. the special landscape area, ancient monuments, natureconservation areas), possible flooding in the Deevalley, and high quality agricultural land (Figures4 and 5).

The survey has resulted in considerablerevision of the existing geological maps. The ten1:25 000 scale thematic maps, ninenew 1:10 000 scale geological maps, reports and computerised databases enable geological factors to be moreaccurately assessed in relation to land-use planningdecisions. However, the study should notbe used as a substitute for on-site investigation.In particular, where coal mining is suspected to have taken place the mine plansand shaftregister held by British Coal should always be consulted.

3 INTRODUCTION

The data summarised in this report were obtained during a21/4 year contract, commissioned in 1988 by the Departmentof the Environment onbehalf of the Welsh Office. Funding forthe work was shared jointlyby the Departmentof the Environment and the British Geological Survey.The study area (Figure 1)is covered by parts of Sheets 108 (Flint),109 (Chester), 121 (Wrexham) and 122 (Nantwich) of the British Geological Survey andlies entirely within the county of Clwyd. The component 1:10 000 maps areshown in Figure 2.

Objectives

The objectives of the project were to provide a coverof modern geological maps for the Wrexham area and, derived from this basic data, to present applied thematic information on geological conditions as they relateto planning and development.The work has complemented and extended information obtained during the Deeside contract(1985-88) covering the areaimmediately to the north. Additionally it has built upon earlier projects commissioned by the Department of the Environment aimed at developingcost-effective methods for computer productionof maps fora variety of user requirements. The mainaims of the project can be summarised asfollows: To carry out, wherenecessary, new basic geological field mapping; elsewhere to reviseexisting geological maps by the use of aerial photo-interpretationwith limited field checking; from this data and other information to producea new set of revised geological maps. To collect and evaluate geological and geotechnical data from site investigation and other boreholes, from shafts and adits, and from other sources and to enter it into computer databases. To present thegeological and other dataas thematic maps and reportsin a formwhich can easily be understood by planners and other interested parties not trained in geology or related disciplines. To develop new and existing computer techniques to allow the various databases to interact within a Geographic Information System so that the final thematic and geological maps and other informationcan be computer generated,in colour where applicable. To identify the need for furtherinvestigations or specialist advice in relation to specific planning and development objectives and proposals.

The useof this report andits limitations

The aim of the report is to provide geological information in a readily comprehensible form and assumes that some readers may have little geological or geotechnical background knowledge. Hence technical jargon is kept to the minimum. Since, however, some technical language is necessary, a glossary of terms is included to assist the reader.

The reportis intended to actas a guideto other more detailed sources, e.g. British the Geological Survey archives of non-confidential boreholes and other data, including, most importantly, the

4 1:lO 000 geological standards and originalfield-slips which are the fundamentalsources on which much of the reportis based. Itis advisable that the maps and report areused not in isolation ofone another. Each map hasonly alimited descriptive key and a fuller detailed descriptionwith relevant provisos is contained in the report.

This report and its constituent maps have been produced by the collation and interpretation of geological, geotechnical and relateddata froma wide variety of sources. Details of these various data sources aregiven in Annex B.

The maps provide only a general description of the nature and extent of factors relevant to the planning of land use and development. The data onwhich they are based are not comprehensive and their quality is variable, and the maps reflect the limitations of that data. Localised or anomalous features and conditionsmay not be represented, and any boundaries shown are only approximate. No information made available after 30th September 1990 has been taken into account. For thesereasons:

This report and itsconstituent maps provide only generalindications of groundconditions and must not be relied upon as a source of detailed information about specific areas, or as a substitute for site-investigationsor groundsurveys. Users mustsatisfy themselves, by seeking appropriate profes- sional advice and carrying out ground surveys and site-investigations if necessary, that ground conditions are suitable for any particular landuse or development. However, the report and maps are a valuable sourceof general information relevant to planningissues and provide a convenient background for thosedesigning site investigations.

All National Grid References in this report liewithin the 100 km square SJ. Grid references are given to eithereight figures (accurateto within 10 m), orsix figures (accurate towithin 100 m) for more extensive locations.

Boreholes and shafts registeredwith the British Geological Surveyare identified by a four element code (e.g. SJ 35 SW/15). The first two elements refer to the relevant10 km National Grid square, the third element to the quadrantof that square and the fourth toaccession the number.

Data, including borehole logs, usedin preparing this report andassociated mapsare lodged at the Aberystwyth office of the British Geological Survey. Any enquiries concerning these documents shouldbe directed tothe RegionalGeologist for Wales, Bryn Eithyn Hall, Llanfarian, Aberystwyth, Dyfed SY23 4BY.

5 Presentation of results

Four reportshave been prepared: 1 Applied geological mapping in the Wrexham area:geology and land-use planning [Thisreport] 2 Applied geological mapping in the Wrexham area-computing techniques (Loudon et al.,1991)

3 Engineering geology of the Wrexham area (Waine etal. , 1990) 4 The swelling characteristics of weatheredCoal Measure mudrocks from Sydallt, near Wrexham, Clwyd, Wales (Entwisle, 1989)

Ten thematic elementmaps (at1:25 000) scale and two thematic elementfigures (at 1:83 333 scale, in this report) have been prepared: Maps 1 Solid geology 2 Drift geology 3 Rockhead elevation 4Drift thickness

5 Mining activities - Coal/Metalliferous

6 Bedrock resources - except CoaVMetalliferous 7 Sand and gravel resources

8 Engineering geology - solid

9 Engineering geology - drift 10 Geological factors for considerationin land-use planning Figures 18Hydrogeology 19 Mineral resource potential

Nine 1:10 000 geological maps have been produced:

SJ 24 NE; 25 NE, SE;34 NW, NE;35 NW, NE, SW,SE

6 GEOGRAPHICAL AND PLANNING BACKGROUND

Geographical background

The study area (215 km2) lies within the county of Clwyd. Its regional context is depicted in Figure 1. The component OS 1:10 000 sheets of the study area areshown in Figure 2, together with the outlines of the former 1:lO 560 County Sheet grid which formed the topographical base for previous geological surveys of the area. Much of the archival material used in the report is also recorded on theCounty Sheet base.The generalised topography, drainagepattern andmajor towns are shown in Figure 3.

The area reachesits highest point (460 m) in thesouth-west, at thesummit of Esclusham Mountain. This high ground, which extends northwards towards R.the Cegidog and thence north-eastto Hope Mountain, reflects the outcrop of the Carboniferous Limestone and the basal sandstone of the Halkyn Formation. In the southern part of the study area, south of the Nant y Ffrith, there is a general easterly slope from Esclusham Mountain towards Wrexham. This is interrupted by a number of escarpments formed by sandstones in the Bettisfield Formation. These are most noticeable in the Brymbo- Gwersyllt area where theCefn Rock forms two parallel ridgesseparated by faulting. In the drift-covered ground from Wrexham eastwards there is, overall, avery gentle easterly slope towardsthe broadvalley of the River Dee.This slopeis interrupted by many minor undulations andshallow stream valleys, and alsoby a marked eastwardfacing slope from Marford to Marchwiel at the eastern edgeof the sand andgravel deposits of the ‘Wrexham delta-terrace,. This southern part of the area is drained by the R. Clywedog and its tributaries. The general drainage is from west to east, but a number of minor streams trend approximately north-south. Moundy topographywith some large enclosedhollows (kettle holes) partlyfilled with clay and peat is well developed between Gresford and Wrexham.

North of the Nant y Ffrith the topography is more complex, reflecting the greater complexity of the underlying geology. The general NNEslope of the terrainis interrupted by the low ground of the drift-covered Leeswood Coalfield between the north-west corner of the study area and Hope Mountain, and also by the NNW trending valley of the R. Alyn.This valley was initiated during late-glacial times between the marginsof the Welsh and Irish Sea ice-sheets, andwas the conduit for the melt-water which laid down the sand and gravel deposits around Wrexham, Llay and Gresford.

The main town is Wrexham, the largest urban centre in North Wales. To the north, west and south-west are the so-called ‘urban villages’, settlements originally based on coal mining and associatedindustries, such asBrymbo, Coedpoeth, Gwersyllt,Llay, New Broughton, Rhosllanerchrugog and Rhostyllen. Other small towns and villages include Bangor-is-y-coed, Caergwrle, Gresford, Hope, Minera and Rossett. The economy of the study area has seen rapid change over the last thirty years as the traditional industries of coal mining, steel making, and associated heavy industries embraced new technology and production methods. Enormous job losses occurred in manufacturing in the late 1970’s. Out of this decline came avariety of positive measures resulting in a more diversified, modern and robust economy andin the alleviation of the

effects ofhigh unemployment and physical dereliction. A major industrialestate is located to the east of Wrexham, while the Wrexham Technology Park offers facilities for ‘high technology’ industries, research and development. Smallernew industrial areas areassociated with many of the other towns and villages.

Two further significant parts of the local economy, namely agriculture and the mineral extractive industry, are likely to continue toplay an important role. Agriculture is largely based on sheep farming on the high ground in the western part of the study area; further to the east, on the drift-covered Coal Measures and Permo-Trias, dairy and arable farmingare more prevalent. The soils on this drift-covered land are very variable; thelight and well-drained soils on theextensive sand and gravel deposits around Wrexham and Gresford contrast with the heavier clay soils developed on thetill-covered ground to the south, east andwest.

The main extractive industriesare limestone quarrying and sand andgravel extraction. They area major componment of the local economy, providing aggregate both for local needs and for the major conurbations of Merseyside, Greater Manchester andelsewhere (see ‘Mineral working in Clwyd’, 1982b). At present limestone is quarried at one site, sandstone at one site, and sand and gravel extracted from threesites. Since the closure of Bersham Colliery in 1987, coal extraction has been limited to small opencast sites, and further opencast productionremains apossibility for the future. The RuabonMarl is dug for tile-making at one site.Fireclay andbrick clays have been dug and minedin the past andthere has been extensive quarryingfor sandstonein several areas. Mining for metalliferous ores,mainly lead andzinc, was formerly an importantactivity in the western part of the study areaclose to Minera.

Planning Background

Geology bears a relationship,directly or indirectly, to a varietyof physical planning considerations, and is thus included amongstthe numerous factorswhich need to be taken intoaccount in allocating land uses to appropriatelocations.

The main factorsinvolved are as follows: Supplies of mineral resourcesare essential to theeconomy but extractionof these can have adverse effects on the environment. Yet minerals can be worked only where they have been formed by geological processes. Planningof land use can direct future extractionto theleast environmentally damaging locations but, forthis to be possible, it is necessary to have general information on the types and extentof minerals present. Applied geological maps provide a broad indicationof this. Conversely, minerals are a finite resource which will be needed by future generations. It is imprudent therefore to sterilise valuable resources by, for instance, building over them. Again mineral resource maps have a part play to in avoiding such sterilisation. Land affected by underground, particularly shallow, mining may be liable to subsidence of the surface with consequent risks to property and people. Similarly some slopes may be subject to landsliding. Ground movements such as these may be triggered by natural processes or by human

11 activity including excavation and construction. Most of these problems can be overcome by engineers but ata cost. Therefore caution is needed when developing or redeveloping land, and consideration of potential ground problems when planning landis prudent.use Appliedgeological maps assemble relevant background information in a form which can be readily used in both planning and in designing site investigations priorto development. Underground water resources need to be safeguarded from pollution from, for instance, waste disposal or industrial sites. The pathways by which contaminants enter the ground aredefined by the natureof the rocks, both unconsolidated andhard, beneath thesoil. Appliedgeological maps highlight the properties of these materials and, thus, give general indications of where problems might occur. Low-lying land in river valleys may be liable to flooding. Developmentin such areas may be atrisk. Maps showingthe extentof such areas can guide thinking on provision the of precautionary works or onallocations of land for future development.

These fourexamples demonstrate the relevanceof applied geological maps to planning, development and protectionof the environment. Twoof the examples relateto earthresources. The others concern constraints which might limit the potential for development or cause higher costs if

development is carried out. The mapscan be used alongside the broader rangeof information - social, economic, ecological, andso onwhich is needed in making decisions on safe,cost-effective and environmentally friendly development.

National planning policies, other strategic guidelines, and Structure Plan policies set the broad framework forplanning at thelocal level.The Structure Plan setsout, in broad terms, the general scale of future development ona countybasis, and thepolicies which will be used to encourage and control it. It is the Local Plan which sets out detailedpolicies and specific proposals for the use of land, guides most day-to-day planning decisions and aims to ensurea balance between the needs of development and environmental conservation.

Guidelines for the preparation of structure and local plans, and their implementation through development control, are set outin a seriesof planning policy guidance notes (PPG’s) and minerals policy guidance notes (MPG’s) issued by the Departmentof the Environment andWelsh Office. Notes of particular relevanceto planning and developmentin the areaof study include those dealing with opencast coal and the provision of mineral aggregates (MPG 3 & 6), and PPG 14 which examines the factorsinvolved in development onunstable land, including land areas which have been subject to shallow mining.

The strategicplanning framework for the study areais provided by the approved Clwyd County Structure Plan (1982a), the submitted Clwyd County Structure Plan: First Alteration(1990) and the Clwyd County Special Landscape Area Local Plan (1984). The study area is covered by the draft Alyn and Deeside (except Broughton and Higher Kinnerton) Local(1984), Plan the adopted Wrexham Maelor Local Plan(1989) and the draft Wrexham MaelorLocal Plan Review (1990).

Th.e general content of these plans is indicated below as a contextfor thesubsequent discussion of geological factors which are relevant to land use planning. Since these plans are subjectto review, however, some policies may be modified or changed. Therefore the reader is cautioned to check the most recent version of the plan at the local authority offices rather than rely on the account given in this report.

Both strategic andlocal planning strategies aimto consolidate the study area’s assets andbuild on past successes to provide for the needs of local residents, businesses, and visitors. Emphasis is placed on improving the local economy. At thesame time, older urban and industrial areas toare be regenerated and environmental quality improved. Maximum use is to be made of existing infrastructure whilst minimising the cost of new provision.

The Clwyd structure plan aims for most development to be concentrated in the existing main settlements, and indicates that the main releasesof housing landin the county are to inbe Wrexham Maelor. The population of Wrexham town and the surrounding ‘urbanvillages’ at the1981 Census was close to 100,000, and continuing population growth is envisaged in and around Wrexham. While the 1982 Structure Plan placed an emphasis on growth in thevillages’, ‘urban the Alteration, in view of the scarcity of suitable sites and related environmental considerations, indicates that a

. degree of expansion is to be allowed for at both Wrexham town and the outlying urban villages. TheAlteration explains that over-rapidgrowth of Wrexhamtown would nonetheless be undesirable, in view of the presenceof high quality agricultural land, especially to theeast of the town; the presenceof valuable reservesof sand and gravel; andhigh quality landscapeto the south of the town.

The Structure Plan and Alteration also set out county-wide policies for mineral working, the protection of high quality agricultural land and protectionof environmental resources.

The policies for minerals identify criteria forthe consideration of proposals for mineral working and provide guidance for the safeguarding of resources and the release of land for extraction, including sand and gravel, limestone, coaland which are of particular significance in the study area.

The plan sets out a policy for Green Barriers, intended to protect the individual identity of settlements and prevent their uncontrolled expansion. In the Special Landscapewhich Area covers much of the higher ground of the western part of the study area, priority is to be given to the conservation and enhancement of the landscape, andin Local Landscape Areasonly appropriate development will be considered. Ancient monuments, for exampleOffa’s Dyke and Wat’s Dyke, are also protected from inappropriate developmentas are Sites of Special Scientific Interest such as those near Minera,at Llay Bog and at Vicarage Moss, Gresford, aswell as other sites of nature conservation interest. The structureplan policies are amplified and given locational expression in the local plans. Boundaries orlimits to settlements are defined, to delineatebuilt-up the areas and main sites proposed fornew development, and also marking the edgeof open countryside.

The major planning factors referredto in the aboveplans are shown diagrammatically on Figure 4.I A further policy aim quoted in the structure andlocal plans is the protectionof the best grades of

14 agricultural land, and is particularly significant in the vicinity of Wrexham. This factor is shown separately on Figure 5, for reasonsof clarity, and thefact that theinformation is derived not from the structure orlocal plans, but theMinistry of Agriculture and WelshOffice’s Agricultural Land Classification. It is to be noted that thebase information used dates-from the 1960’s, and has been subject to partial revision; howeverthe figure is intended to providea diagrammatic representation of the distribution of the higher grades of agricultural land in the study area.

With respectto water resources, the National Rivers Authority controlsthe quality of water under the Control of Pollution Act 1974, the Public Health Act 1937 and the Water Act 1989. The Structure Plan Alteration pointsto the need avoidto any formof development which would cause the discharge of effluent in a mannerlikely to impair thequality of coastal or inland water.It follows that protection of the major aquifers of the area from, for example, industrial and toxic waste disposal is an important factor in terms of planning and development. A further environmental consideration arises fromthe ‘Deeflood plain safeguarding areas’ (Figure4), covering thoseparts of the Dee valley where flooding could take place if the riverside levees were to be breached or overtopped by flood water.

In general, itis clear that considerationis required to begiven to all factors affecting development costs, stability of structures, and safety. Hence, the geotechnical characteristicsof deposits likely to be encounteredduring development needto be bornein mind, and any adverse impact related to geological features minimised. Such constraints are considered in more detail in the Summary of Geological Factors for considerationin Land-use Planning.

GEOLOGICAL BACKGROUND (THEMATIC MAPS1-4)

Introduction

Maps 1-4 provide a background to the geology of the study area; most of the information on the subsequent Thematic Mapsis derived directly or indirectly from them.A more detailed account of the geology of the areais given in Annex A. TheSolid and Driftgeology maps (1 and2) are derived directly from the1: 10 000 geological maps.

The geological deposits of the areacan be divided into tworadically differing categories. Theyare, firstly, the solid (or bedrock)sediments (Figure7), mainly mudstones, sandstones and limestones, all of which are indurated ato varying degree. They rangein age from Ordovicianto Permo-Triassic (some 440 to 210 million years old). Over much of the area thesolid deposits are overlain by the second category, unconsolidated drift(superficial) deposits (Figure 8). These, mainly clay, stony clay, sand and gravel, form a coverof variable thickness (Figure11) and have been deposited during the last 20,000 years.

Solid geology (ThematicMap 1)

The mapdepicts the surfaceof the solid bedrock, which in manyplaces is covered by drift deposits (Map 2), and shows the distribution of the major rock formations. It does not attempt to portray the vertical variations within the solid rock, other than in the generalised vertical section and cross-section.

The ‘formation’ is the fundamentallevel of subdivision of a sequenceof rocks and refersto a body of strata which is recognisable and mappable over large areas.The formationcomprises strata of distinctive geological characteristics (e.g. similar composition, grain-size, sedimentary structures, persistentassociation of beds of differentrocktype). Specific lithologiescan commonly be recognised within a formation (e.g. sandstones, coal seams) and may be named as a ‘member’. These needonly beof local distribution.

The rock succession of the areais given in the formof a generalised vertical section. Thisshows the stratigraphic orderof the formations, their thickness variations (approximately to acrossscale) the area and general relationships with underlying and overlying formations (i.e. normal contact or unconformity). Brief descriptions of each formation and member,including age, rocktype, lateral and vertical variations, and interpretationof mode and environmentof deposition are given in the Summary of Geology (AnnexA). The cross-section is an interpretation,based on available surface information and relevant borehole data, of the natureof the solid rock at depthalong the line of section shown onthe map. Theline of cross-section was chosen to best illustratethe overall structure of the area.

This map should be used in conjunction with Map 2 (Drift geology), asthe accuracy and certainty of the linework depends to a large degree on the availability of information from rock exposures and the thickness of the drift. Thusin areas of little or no drift alesser degree of interpretation is necessary. However, in the coalfield, considerable information is available, despite a widespread drift cover, from boreholes, previous mining and opencasting. Therefore, in some areas, the position of coal seams and faultsis wellconstrained.

Faults are discontinuities in the solid rock and may,in addition tooffsetting geological boundaries, coincide with changes within the formations. Most faults in the area are steep (within 20” of vertical). No attempt is made toindicate the variation of steepness exceptin the cross-section. Few faults are everseen in exposure other thanin mines and quarries and, therefore, theposition of most of them is inferred.

Most of the study area is underlain by sedimentary rocks of Carboniferous age. Theselie unconformably on Ordovician strata, and are themselves overlain unconformably by Permo-Triassic strata. The Carboniferous sequence has,its base,at up to225 m of Carboniferous Limestonewhich is mainly limestone with subordinate mudstones and sandstones. Thisis overlain by up to420 m of Halkyn Formation (Millstone Grit) sandstones, siltstones, mudstones and chert. Thesein turnare succeeded by up to 520 m of the Bettisfield Formation (Productive Coal Measures) which is characterised by cyclic repetition of mudstone, siltstone, sandstone, seatearth andcoal. The Main

18 coal locally exceeds 4 m in thickness and a numberof other coals are more than3 m thick in places. The Productive Coal Measuresare succeeded by up to about900 m of ‘Red Measures’(Upper Coal Measures). These also commonly show a cyclic repetition of strata, but coal seams are poorly developed. The succeeding Permo-Triassic sandstones (up to540 m) areentirely covered by thick drift deposits, of glacial or post-glacial origin, as is most of the outcrop of the Productive Coal Measures and ‘RedMeasures’.

The most significant structural featureis the ENE-trending Bala faultsystem (Figure 7). It has a long and complex history, possibly since Precambrian times, and there aresignificant differences in the Carboniferous sequences on either ofside it (AnnexA). It does notaffect the Permo-Triassic rocks, althoughit can be tracedby geophysical methods for some distance under the unconformable Permo-Triassic cover.

On the southernside of the fault system the stratahave a general gentle dip towardsthe east(see Horizontal Section), and in broad termsthis is reflected in the topography. There area numberof major NNW-trending faults (such as the Gresford and Wrexham faults) with a subsidiary E-W- trending set. To the northof the fault system the structureis more complex.Lead, zinc and copper mineralisation is largely confined to the Minera area, to the southof the Bala fault system. This mineralisation occurs mainly as discontinuous veins along NW-SE-trending faults and joints, and in some steeppipes.

Drift geology (Thematic Map2)

This mapshows the distribution at surface of all drift (superficial) deposits.The general term‘drift’ is applied to the unconsolidated,superficial deposits of the most recent (Quaternary) age, and these are largely glacial in origin or are relatedin some way to theglacial period which ended some10,000 years ago. Localised peat and clay-filled depressions related to the deglaciation, and known as kettle holes, are symbolised. Deposits which have formed recently, or are actively forming by natural processes are also shown, together with man-made deposits (made ground, backfill and landscaped ground). The mapalso displays landslips of various ages (Quaternary to Recent).

The drift andother unconsolidated depositsare categorised accordingto two factors, a) theprocess by which they were formed, and b) the composition of the deposit. The formerincludes landslip, macle ground, backfill, landscaped ground and the various forms of alluvium. The latter includes till (boulder clay), sand and gravel, glaciolacustrine deposits and peat. Those categorised solely according to processare thereforevery variable in grain-size, clast content and fabric.

The mapshows only the variation of surface deposits(i.e. within 1 metre of surface) and makes no attempt to show variations with depth. Although the depositindicated on the map may be considerably thicker than 1 metre and perhaps theonly deposit overlying the solid rock at a given site, it is equally likely that theremay be many different typesof deposits. Therefore, borehole data must beconsulted (Annex D). Driftthickness variations are shown onMap 4, and further information in the Summary of Geology (Annex A).

Deposits generallyless than 1metre thick are not shown. Areasshown as solid rock (bedrock)may have extensive veneersof drift. Itmay locally exceed 1 metre in thickness, but such areas are too patchy to show. Thin drift veneers may, however,be avery important factorwhen considering local ground stability (Waineet al.,1990). Much of this veneerwould be classified as ‘head’, a product of weathering commonly combinedwith solifluction and hill-creep. Head has not beenshown as a separate deposit on the geological maps or on this Thematic Map; where it is of appreciable thickness it is included with alluvial or glacial deposits.

The categories of drift and other unconsolidated deposits are briefly reviewed below.

Landslip There arelandslips in both bedrock and superficial deposits. Some of these landslipsstill active are [e.g. SJ 3260 48441. Most of the landslips recognised are probably of immediate post-glacial age and may not have beenactive for thousandsof years. It must be stressed, however, that excavation or loading of recorded landslips, and particularlythe fronts landslips,of is likely to reactivate them. Most landslips occur along steep-sided valleys, and are particularly common where the Halkyn Formation or Coal Measures are the bedrock, or where thickthere driftare deposits.In such valleys landslips may be reactivated or triggered by active stream erosion. Even wherea landslip has not been mapped it should be emphasised that any of these steepvalley sides are potentially unstable. Examples of landslipping are along the valleys of the R. Cegidog [SJ 288 548 - 308 5501 and R. Alyn [SJ 323 5491.

Made Ground Most areas of made ground are related to mining (metalliferousand coal), quarrying, land development and road andrailway embankments. Thelargest areas are thoseassociated with the Minera lead mines [aroundSJ 261 519 - 277 5091, Brymbo Steelworks[SJ 298 5341, the deepercoal mines in the eastern partof the coalfield for example atLlay Main Colliery [SJ 325 5621, Bersham Colliery [SJ 312 4801, Hafod Colliery [SJ 311 4691 and Gresford Colliery [SJ 336 5381, and along the A 483 trunk road.Most made groundis only up toa few metres thick, buttips associated with the deepercoal minesmay exceed 35 m in height.On the mapa distinctionis made between made ground related tomining activities and other made ground; further informationis given on Map5 (Mining activities - CoaVMetalliferous), Map 6 (Bedrock resources - except CoaVMetalliferous) and Map7 (Sand and gravel resources).

Metalliferous mining waste tips generally comprisecalcite vein material and thelocal country rock, principally limestone. Some of the tips, however, may contain a significant content of sulphide minerals(galena (lead) and sphalerite (zinc)). Coal mining tips usually comprisemudstone, siltstone and sandstone debris, and sometimeshave an appreciable content of coal. Some of the larger tips (e.g.Llay Main) are gradually being removedor landscaped and their contents usedfor the manufactureof breeze blocks and similar products. Made ground related to limestone quarrying generally comprises calcareous mudstone and muddy limestone, while that related to sand and gravel extraction usually consists of fine-grained sand, silts and mud.

22 Backfill of sand and gravel, opencast coal and other extraction sitesusually comprises the waste products of the extraction (e.g. sand and gravel[SJ 357 5601, coal [SJ 307 4991, clay [SJ 309 4541) as given for made ground above. Itmay also include, however, household rubbish and industrial waste at approved sites (Figure18). On the mapa distinction is made betweenbackfilled opencast coal sitesand othertypes of backfill.

Landscaped ground Landscaped ground refersto areas where originalthe topography has beenextensively remodelled by earth-moving and tipping and whereis itimpracticable or impossible to delineate areasof made ground. In the presentstudy its use has been restricted to the Wrexham Industrial Estate where most of the land falls into this category. Most of the area of the estate was originally a military establishment with installations both above andbelow ground; these havemostly been demolished and the area landscaped for industrial use. Further landscaping is commonly undertaken as industrial development takes place.

Alluvium Alluvium (floodplain deposits) occurs along most of the more important streams and rivers. It occupies a considerablearea along the Deevalley both in the south-eastern cornerof the study area near Bangor-is-y-coed and in the north-east around Trevalyn Meadows. There are significant stretches of alluvium along the Alyn valley upstream from Caergwrle and downstream from Gresford. Alluviumis still actively forming alongside many streams and rivers as ofa periodicresult flooding. It consists mainly of fine sands, silts and clays, with local gravels; commonly sands and gravels in the lower part of an alluvial deposit are overlain by silts and clays above (seealso Map 7 - Sand and gravel resources).

Alluvial Fan deposits are of limited extent and have been includedwith alluvium on Map2. They are shown separately on the1:lO 000 geological maps. They form where constricted streams and rivers flow into a less constrained area (e.g. thealluvial floodplain of a large river) andthe sediment carried by the streamis laid down in a deposit which increases areally downstream.There may be a pronounced grain-size decrease downstream on the fan (e.g. gravelvarying downstream to fine sand and silt).

Alluvial Terrace Alluvial terraces are best developed in the southeastern and northeastern parts of the area, adjoining the alluvium of the River Dee and thelower reaches of the River Alyn. Elsewhere they are localised and of small scale. They comprise similar material to other alluvium, though commonly with a higher sand and gravelcontent. They representflooding during formerly higherlevels of the stream orriver (see also Map 7 - Sand and gravel resources).

Lacustrine Alluvium These deposits are very limited in extent, occurring onlyin a few kettle holesin moundy sand and gravel terrain. They werelaid down in small temporary lakes and ponds frompost-glacial to recent times. They are notdistinguished from alluvium on the Thematic Map.

23 Till (BoulderClay) This is a very widespread deposit. It largely covers a broad tract from Bersham and Minera southwards to the southernmargin of the study area, and thenceeastwards to the Deevalley and north through the Wrexham Industrial Estate to thevalley. Alyn It also covers a largearea between Hope and Burton, and blankets much of the Leeswood Coalfield. It is extremely variable in composition and was the productof direct deposition from meltingglaciers. Clasts of varying size up to several metres are suspendedin a clay matrix. There is usually no order orbedding though occasional beds of sand, gravel andsilt may be intercalated.

In the area, there weretwo different sourcesof boulder clay, with quite different suitesof clasts, clay matrices and grain-size characteristics (see Summary of Geology (Annex A) for further details).

Glaciolacustrine Deposits These deposits occur beneathglacial sand and gravel to the south-east and eastof Wrexham and adjacent to the Wrexham Industrial Estate. They are also seen in the sides of the Alyn valley between Bradley and Gresford. They comprise beddedsilts and very fine sands and werelaid down in an ice-dammed glacial lake (see Annex A). They are difficult to differentiate in the field from other glacial deposits and are probably more extensive than indicated on the map (see Map 9,

Engineering geology - drift).

Peat Hill peat (generally less than 2 m thick) is present on the easternslopes of Esclusham Mountain, west of Fron-deg. Small areas of thicker peat occur in some kettle holes.

Rockhead elevation (Thematic Map 3)

Rockhead elevation is the height relative to Ordnance Datumof the surface of the bedrock. Thus the rockhead elevation contours, as shown on this map, are contours on the topographicsurface which would be revealed if all drift (superficial) deposits were stripped off.It follows that in areas where bedrock is at or very close to the groundsurface (shown by a grey stipple on the map) the rockhead contours and present-day topographic contours will be coincident. Figure 9 displays a simplified version of the contours.

There are sometimes problemsin defining the exact heightof the rockhead where the bedrock has an appreciable zone of weathering. This is most noticeable in the mudstones of the Halkyn and Bettisfield formations where the weathered zonemay be several metres thick.

The rockhead elevation contours have been computer generated (Loudon et al., 1991) from two main sources. Chief of these is the borehole database (Annex D), and both confidential and non-confidential boreholes have been used in their compilation. Information comes from those boreholes which have a reliable depth forthe rockhead, and from thosewhich fail to reach bedrock but nevertheless provide a maximum height for the rockhead. Additionally the contours are constrained by the coincidence of the drift margins with the topography. Borehole evidence and

38 38Q 1 SJ24NE Q 38R 1 SJ24NE 1 I U-AFONEITHA ROAD PEN Y CAE BH5 U-AFONEITHA ROAD PEN Y CAE BH6

Date: 3 1:OS: 1973 Date: 1:09:1973

Grid Reference: 28650 45260 Grid Reference: 28650 45230 Scale: m m 1:200

CLAY

...... SDST YPETH w= MDST

MDST COAL

COAL COAL 1NDST MDST 1COAL MDST COAL - MDST 2-4, COAL SLST MDST COAL MDST 2 SEAMS SEAT YDST COAL SEAT

COAL MDST MDST COAL, 2 THIN COAL SE

TD 18.80

RH Rockhead TD = Total Depth

Figure 10 Examples of computer-derived graphic borehole logs

26 outcrop information from a2 km wide strip outside the margin of the study areawas also used in order to define more closely the rockhead elevation near the margin. Borehole sites, for reasons of confidentiality, have not beenshown on the map; however,give to an impression ofthe reliability of the contours aninset map of borehole density has been provided.As well as producing rockhead elevation contours the borehole databasecan be used to generategraphic vertical sections (Figure lo), three-dimensional diagrams and other plots.

The formof the rockhead contoursparticularly indicatesthe formof buried channels relatedto the major river systems and elsewhere. Inthe eastern partof the area the rockhead contoursshow a steady eastward slope towards the buriedvalley of the pre-glacial River Dee. Borehole controlis poor in this area, especially since few boreholes reach bedrock, but information from the study area and surrounding areasindicates that, in its northern part, thepre-glacial Dee valley had a NNW trend approximately through Ridleywood[SJ 399 5181 and Trevalyn[SJ 380 5671, with its basewell below Ordnance Datum.Local smaller scale sub-drift, andin many cases sub-glacial, channelling is seen in a numberof places, notablywest of Rhosllanerchrugog and adjacentto the restoredPlas Power opencast coal site.

Drift thickness (Thematic Map4)

The drift thickness contours, or drift isopachytes (simplified in Figure 11) were obtained by interacting the rockhead elevation contours with the Digital Terrain Model (DTM). The DTMis a digital record of the topographic contoursof the area. Thethickness of drift at any point is the difference in elevation between the topographic surface and the rockhead surface and a contoured map of drift thickness canbe obtained by subtracting the heightsgiven by the rockhead elevation plot from those of the DTM (details of this process are given in Loudon et al.,1991). They also emphasise the generalincrease in drift thickness towardsthe eastof the area,but additionally they highlight the considerable local variation within this general trend.

A knowledge of the approximate rockhead height and the drift thicknessis of great value when planning site investigations. Maps 3 and4 can be used in conjunction with Map 2 (Drift geology) and Map 9 (Engineering geology - drift), and the information contained within the Borehole Database (Annex D), to obtaina preliminary assessment of ground conditions.

MINING AND MINERAL RESOURCES (THEMATIC MAPS5-7)

Mining activities- Coal/Metalliferous (Thematic Map5, and Figure12)

This mapshows the areasmined for coal and metalliferousore, andaspects relatedto thatmining. Some of the early mines were sunk for ironore (siderite) rather than coal, and somespecifically for the extractionof fireclay (seatearth). The information coalon mining is derived to a large extent from that of the British Coal database (Opencast Executive, Coal Commission and other abandoned mine plans) and BGSarchival data. Theinformation on metalliferous mining is derived from previous surveys of the area with supplementary BGS archival plans and maps. The map gives a broad picture of where undermining has, or may have, taken place, and consequently gives an indication of areas where landinstability may occur. In the case of specific site investigations, the documents heldby British Coal and atthe Aberystwyth Office of the BGSshould be consulted.

Coal Mining Coal has been mined in the area at least since the Middle Ages and possibly earlier. Earliest workings were likely to have been of coal at outcropin the western part of the coalfield, gradually developing into shallow workings by adits and bell pits. The concentration of shafts related to shallow workings around Rhosllanerchrugog is particularly striking. With the advent of greater demand for coal during the industrial revolution, deeper mining became prevalent and steadily extended eastwards under anincreasing cover of ‘red measures’ (Ruabon Marl andhigher beds). The deepestworkings, at the eastern edgeof the worked area,reached depthsof more than1000 m below OD. Deep mining in the areaceased with the closure of Bersham Colliery in 1987.

The mapdelimits the areasof underground workings for which plans are available. Prior to 1872, there was no statutory obligation to record plansof underground workings. Theonly information relating to very early mining derives from surface expressions(e.g. shafts, depressions, waste tips) where recognisable. Even after 1872, the plans lodged with the Mines Record Office (copies of which are held by British Coal) wereof very variable quality and accuracy.Few of the olderplans record details of depth of workings and many show little surface detail by which they can be positioned. On some plans, for example,it is clear that the direction of north shown is inaccurate, and plotting such information is therefore liable to considerable error. Plans for the more recent deep mining in the eastern partof the coalfield (e.g. at BershamColliery) are detailed and accurate.

The areasof underground workings shown are differentiated into areas whereonly one coal seam has been worked and those where more thanone seam has been worked. Approximately60 km of the areais known to be undermined.No attempt is made to indicate the depthsof workings since insufficient information is available for much of the area. It is certain that the records of underground workings are incomplete. Thus, known mine-shafts, adits andcoal waste tipsare shown in areas without any mine plans. The possibility exists, therefore, of shallow workings anywhere in the area where theBettisfield Formation (Productive Coal Measures)is at or nearsurface (Map 10) and particularly so where the thicker seams (e.g. the Main and Quaker coals) occur at outcrop or shallow depth. Thelikelihood of shallow workings will decrease with increased superficial overburden.

Many of the mine-shafts and adits shown were recorded during the various surveys of the area. Additional sites have been transferred from British Coal(which plans indicate the validation of the data) andall have been checkedagainst the British Coal Shaft Register. Evenso, the exact sites of many of the shafts shown are uncertain. Some1350 shafts and adits are recorded,but unrecorded shafts are likely to exist. Some shafts have beentreated by capping or filling. However, it cannot be assumed that treatmentcarried out in the past remains effective, or thatit was undertaken to standards suitable for future specific land uses. Uncapped shafts may be wholly or partly filled. Therefore all shafts needto be located, their condition determined,any and necessary precautionary works undertaken before developmentof the land.

It cannot be stressed too strongly that shallow workings, shafts and underground workings may exist in addition to those shown on the map, and that limitsthe of underground workings are only approximate. Unrecorded shafts, in particular, are a hazard and are liable to collapse without warning.

There is little site-specific information about the methodsof underground workingused in the older mines. However, it is known that two general methods were employed. The earliest techniqueof large-scale extraction was the ‘pillar and stall’ method. This involved the selective extraction of coal, leaving up to 60% in position to support theroof of the seam. Subsequent improvementsin technique increased the efficiency of recovery. Thus panel working and longwall methods were used from the latter partof the 19th Century onwards (and probably appliedin some cases to seams already minedby pillar and stall).Many of the available mine planssuggest longwall extraction was used. This involved the complete removal of coal from certain areas resulting in the controlled collapse of the unsupported seam roof. On the map, no attemptis made toshow areas of potential subsidence due to mining. This would relate to the age and depth of workings, and number of worked seams andthe thickness of superficial deposits. However,the map doesshow areas where the drift cover is greater than 30 m thick since, in general, the effects of collapse of old workings are largely masked by overburden of that thickness.

Of the two main methods of mining, areas of pillar and stall providethe greater potential hazard to development. As a large proportion of the seam was left as roof support, a borehole is highly likely to pass through the pillars (i.e. the preserved seam) rather than voids. adjacent Furthermore, downward bowing of the seamroof between pillars may suggest in a borehole thatonly a thin seam is present, whereas voids close to thepillars may still exist. Collapse of the pillars themselves can result in cavities, breccia pipes andvoids in overlying strata. Inmost parts of the coalfield any voids are now likely to befilled with water.

Longwall extraction, used generally for the deeper seams,typically results in the collapse of the unsupported seam roof during or soon after mining and further collapse is unlikely. Only the connecting ‘roads’which were permanently supported foraccess are likely to cause later subsidence problems and problemswith voids. Most of these workings in the western part of the coalfield (i.e. where the Bettisfield Formation is at surface or crops beneathdrift) had been abandoned by the 1930’s and consequently little subsidencedue tocollapse is likely. Recent mining has been confined to the deeper partof the coalfield to theeast under athick overburden of ‘red measures’ and drift

31 at collieries suchas Bersham and Gresford. Here the depthof the workings, addedto thediffusing effect of the drift , mask any subsidence.

The most extensively worked seams are the Main and Quaker coals (see Summary of Geology, Annex A). The total areawith Bettisfield Formation (Productive Coal Measures) at surface or at crop under drift is about 50 km. Under the ‘red measures’ the worked area extends to a further 45 km. The cumulative average thickness of all the coal seams is some 20-25 m. The original ‘resources’ of the area wereof the orderof 1300,000,000tonnes. Of this total, the extentof recorded workings suggests that approximately 30-40% of these resources have been substantially extracted. The remaining ‘resources’ are not necessarily readily available (e.g. due to unrecorded mine workings, substantial overburden, local thinning and washouts of coal seams), or amenable to modernextraction methods. Furthermore, no attempt canbe made to assess the quality of individual seams. As coal seam nomenclature varies across the area, a ‘correlation’ of the local seam nameswith the nameused by British Coal for thewhole of the NorthWales Coalfields is given in Table 1.

Backfilled opencast sitesare shown,as are areasof made ground (waste tips) relatedto coal mining. Only one opencast site is active at present, at Cae-110; recent opencasting near Llay Hall [SJ 316 5491 was completed in 1989 and is now backfilled and restored.

Iron Ore Iron orein the formof nodules and, less commonly, thin layersof siderite (impure iron carbonate) is widespread throughout the Bettisfield Formation. It was worked extensively in the 19th Century at such places as Brymbo and Ponciau, generally in conjunction with the mining of coal (Wedd, Smith and Wills, 1928). The industry has been extinct for many years though the steelworks at Brymbo continuedin production until 1990.

Fireclay Fireclays, mainly as seatearths tocoal seams, occur throughout the Bettisfield Formation. Several have been workedin the past for refractory materials (Geological Surveyof Great Britain,1920b) the main locations being at Llay Hall Brickworks [SJ 318 5511 and Cae-llo Brickworks (Smelt Pit) [SJ 288 5431.

Metalliferous Mining Metalliferous mining was restricted to the west of the area around Minera and was largely concentrated on the uppergeological formations of the Carboniferous Limestone (see Summary of Geology, Annex A - Mineralisation). The lead ore galena was the main economic basis for the industry with most production between 1855 and 1880 and a minor revival in 1900 (Smith, 1921; Earp, 1958). Silver extracted from the galenawas a valuable by-product. Thezinc ore sphalerite was not mined in appreciable quantities until 1865, with most production concentrated between 1880 and 1895. Mining had effectively ceased by 1920. As little of this mining was recorded, no attempt is made toshow areas of underground working. Unlike coal mining, however, the ofareas mining were mostly restricted to narrow, relatively thin veins and joints which tend to occur in conjugate sets. As theseveins can, in many instances, be well defined by the linear chainsof shafts

33 Table 1 ‘Correlation’ of Coal Seam nomenclature for the Leeswood and Wrexham areasof the NorthWales Coalfield, after British Coal

WholeCoalfield Local names Name Leeswood Coalfield Wrexham areaof ‘Denbighshire Coalfield’

AlphaRider to Delta - Alpha Rider Bench GroupYardBersham to Delta Bench Gwersyllt Little - Gwersyllt Little or Little Upper Stinking - Upper Stinking, Top Droughy, Cefn, Droughy or Top Four Foot Cannel - Cannel Pontybodkin Pontybodkin Mountain, Mountain Mountain or Top Warras - Warrras, New or Ribbon John 0’ Gate Pontybodkin Divided, John 0’Gate, Sulphur,King or Divided or Mountain; Thomas Evans Tryddyn Half Yard or Half Yard Lower Stinking - Lower Stinking, Bottom Droughy, Droughy, Yard, Four Foot or John The Gate Smiths - Smiths or Top Yard Drowsell - Drowsell, New Century orCrown (bottom leaf only) Powell Powell Powell or Yard Hollin Hollin, Two Yardor Lit- Hollin, Two Yard,King or Seven Foot; tle (bottom leaf only) Two Yard Bench (bottom leaf) Crank Crank Crank or Yard Quaker Brassey Quaker, Brassey, Brass Vein,Five Foot or Four Foot Black Bed Rough, Black Band or Black Bed, Pin,Pin and Shale or Black Black Vein Main Bind Main Bind Main Bind Main Main Main Main Bench Main Bench or Little Main Bench Lower Bench Lower Bench, Two Foot Pin, Little, Brassey or Crown or Finger Crown Diamond, Five Foot Six, Crown, Yard, Upper Yard, Four Foot or Jubilee Little, Crank orNew Upper Red King Upper Red, Red or Crank Lower Red Cannel, Two FootSix, Lower Red, Red,Two Foot or Stone Three Foot, Stinking or Little Stone Stone, Five Foot or Four Stone, New, TopFireclay: Foot Fireclay , Cannel , Middle Stone or Half Yard Half Yard - Half Yard, Bind, Bench, Fireclay New or Strange Group Fire Damp - Fire Damp, Bench, Stone, Nant I

34 ~~ ~ WholeCoalfield Local names Name LeeswoodCoalfield Wrexham area of ‘Denbighshire Coalfield’ Nine Foot Rider Nine Foot Rider - Nine Foot Nine Foot Nant Rider, Nant orWall and Bench Nant Yard Nant, Stone, Wall and Bench, Four Foot or Two Yard Ruabon Yard Cannel Ruabon Yard, Lower Yard,Top Queen, Prince, orWall and Bench Premier Wall and Benchor King Wall and Bench, Queen orWallsend Llwyneinion Half Half Yard, Queen or Llwyneinion Half Yard, Half Yard, Yard Lower Queen Lower Yard, Lower Queen Kingor Chwarelau - Chwarelau

35 and shallow trial pits, displaying the veins themselves provides a good guide to the undermined areas. However,in the case of the deeper,usually more scattered shafts, these were necessarily not sited directly onthe vein trends, and hence the positioningof the veins is subject to inaccuracy. A further problemis that many of the veins depart significantly from vertical, increasing the area of potential undermining.

The mapshows the known shafts, some200 in number; there arelikely to be otherswhich are now obscured. Some shafts have been capped, othersclosed are at surface(due tocollapse) butmay be open at depth. Some estimateof the depth of some of theclosed shafts may be gained from the amount of spoil material surrounding the shaft site. These areasof spoil form ring-like ramparts around the shafts,which are themselves usually preserved as depressions. The larger the volume of spoil relative to the diameterof the shaft-site, the greater is the likelihood of it having been a deep shaft, rather than an ephemeraltrial pit. It is increasingly common practice to attempt land restoration in areas of previous lead mining by bulldozing flat the made ground.Consequently in some areas, the number anddensity of shafts will remain unknown. A computer database containing information about thecondition of known shafts at surface has been preparedby Clwyd County Council.

Ores are principally restricted to veins, joints, pipes and some bedding planes (see Summaryof

Geology, Annex A- Mineralisation). They occur as discontinuous depositswhose distribution and concentration are unpredictable. As modern large-scale base metal extraction tends to rely on prospects of predictable, albeit often relatively low, concentrations, the ore deposits in the area would seem not to be a viable prospectunder present or foreseeable economicconditions. Furthermore, the extensive shallow-level removal of ores during the past would necessitate that future mining would have to exploit resources at somedepth andthis would pose problemsof mine drainage.

Bedrock resources - except Coal/Metalliferous (Thematic Map6, and Figure14)

This map shows the distribution of areas within which bedrock resources, apart from coal and metalliferous deposits (Map5) , are orhave been extracted and where potentialmay exist for further exploitation. Whereapplicable, the resource areasare shown as beinglimited by drift depositsmore than 10 m thick since it is considered that most bedrock resources except coal and metalliferous deposits are unlikely to be exploited beneathan overburden which exceeds that thickness.It must be emphasized that this 10 m limit is only approximate as there are some partsof the study area in which there is insufficient borehole information or field evidence to permit the drawing of an accurate 10 m drift isopachyte.

Active quarries and pits and the larger disused quarries and pits are shown; sites of completely backfilled quarries and pits (where known) are also shown since these could represent a possible hazard for developers. Each quarry or pit, whetheractive or disused, is annotated to indicate the type of resource which was obtained from it; the same annotationis given with each headingin this description. Afew of the disused quarries andpits lie outside indicated resourceareas. These either

37 Table 2 Summary of mechanical and chemical property data from limestones (Adapted from Harrison and others,1983)

Formation Rock type Purity Mechanical properties (generalized) Purity Insoluble AAVFlakiness ACV AIV residue Index residue

Cefn Mawr Dark argillaceous Generally 1.2 - 31.9 27 - 51 20 23- 24 - 25 5.0-9.1 limestone with low purity shales Loggerheads Massive pale grey Very high 0.2 - 1.5 14 - 31 20 25- 25 10.2 - 11.8 limestone purity lie within resource areaswhich are toosmall to show on the map, were or for clay, a resourcewhich is not shown in detail on the map(see under Clay).

Limestone (L) There are two main outcrops of the Carboniferous Limestone, which has a total outcrop area of some 5 km2. The first occupies the structurally complex southern side of Hope Mountain and extends westwards alongthe northernside of the Bala faultsystem through Ffrith toBlack Wood. The second main outcrop lies to the south-west of the Minera Fault, on thenorth-west slopes of Esclusham Mountain. Thisarea is the northernextremity of the limestone outcrop which extends southwards alongEglwyseg Mountain to theVale of Llangollen. Additionally, there aresmall areas of Carboniferous Limestonewithin the Bala faultsystem between the southern endof Black Wood [SJ 265 5401 and the western edge of the study area.Most of the outcrop is free from significant drift deposits.

Three divisions (formations) have been recognisedwithin the Carboniferous Limestone, and their areas of outcrop areknown in detail. The sequenceof formations, their thickness variation, and a detailed description of the rock types in each formation are given in the Summary of Geology, (Annex A). Physical properties of the limestones and estimates of their purity, derived from Harrison and others(1983) and related to thenew stratigraphy, aregiven in Table2.

The limestones of the Cefn Mawr Limestone and Minera Formation have been grouped together on the resource mapsince they havesimilar characteristics. The significant difference betweenthe formations is that the Minera Formationincludes interbedded quartzitic sandstoneswhich are not present in the Cefn Mawr Limestone. Thelimestones of the Cefn Mawr Limestoneare interbedded with mudstones, with the proportionof mudstone increasing towardsthe topof the formation. The Minera Formation, which also includes interbedded mudstones, has been worked in the past for limestone and sandstone; the Cefn Mawr Limestone is at present worked by Tarmac Roadstone Ltd (North West) near Minera[SJ 251 5191.

The Loggerheads Limestone, in contrast to the Cefn Mawr,is of very high purity and has a very low mudstone content. Togetherwith the Cefn Mawr Limestone itis quarried near Minera.

Limestone is without doubt thesingle most important mineral resourcewithin Clwyd. In thestudy area, however,resources are limited since the main limestone outcropslie to thewest on Eglwyseg Mountain and also to thenorth-west between Llandegla and Prestatyn (see Campbell and Hains, 1988). Additionally, in the Minera district,the limestone is thinner than on the northernside of the Bala fault system or southwards towards Llangollen. The underlying Ordovician rocks have been reached in some areas of the quarries and further scope for deepening them, especially in their western parts, is limited. In the Hope Mountain area thelimestones are thicker, but theextensive structural complexity of the areamilitates against large-scale extraction; also much of the outcrop lies within the Minera Formationwith its high content of sandstones.

The amount of theoreticallyworkable limestone in the area (i.e. above the water table) is approximately IO9 tonnes. In practiceonly a very small fraction of thisis likely to be worked and

39 many factors effectively reduce the amount of the resource actually available. These include geological factors such as the presenceof shale and sandstoneunits in the upper partof the sequence and past mining for metalliferous deposits,areas sterilised by housing, and environmental factors such as the effect on topography. Nevertheless, less than 4% of the total volume has so far been extracted and one important resultof this studyis that thenew detailed geological maps will enable the appraisal andfuture exploitation of the resource to carriedbe out moreeffectively now that the areal extent and thicknessof the various limestone typesis more closely defined.

The Loggerheads Limestone is of predictable and constant nature; it is of high purity and good aggregate quality and can generally be expected to be of sufficient strength and durability to be used as a roadstone (base and sub-base) and concrete aggregate (Harrison and others,1983). The Cefn Mawr Limestone and Minera Formation contain limestonesof similar aggregate quality but they also contain ahigh proportion of mudstone/shale waste; however,the association of limestone and shale can be particularlyuseful for cement production.The Minera Formation, however, has the additional problem that sandstonebodies within it detract from its value as a primary resource for the cementindustry.

The most important clay resources are those within the RuabonMarl Formation, thoughclay has been worked at horizons within the Bettisfield and Erbistock formations. Additionally,there have been very small local workings in Till (Boulder Clay). Only the Ruabon Marl Formationclays are shown as a resource on the map. Theonly working pit is at Hafod [SJ 309 4551, near Johnstown, operated by Dennis Ruabon Ltd fortile-making.

The Ruabon Marl Formation is some 200 m thick in the Hafod area,but only the red and purple mudstones in the lower and middle parts of the formation are of economic value. Nearly all the outcrop is obscured by thick drift deposits and workable resourcesare limited to the area around Hafod and to the south of the study area. Similar red and purple mudstones in the Erbistock Formation have been worked bricksfor and terracotta goodsat theWilderness Pit[SJ 335 5351near Gresford and nearKing’s Mills, Wrexham. The Erbistock Formation has not been included as a resource asthere is a great varietyof lithological types within the formation andalso because almost all the outcropis covered by thick drift deposits.

Mudstones and thinfireclays (See descriptionof Thematic Map5) occur throughout the Bettisfield Formation and also in the Halkyn Formation. They have been exploited for brickmaking, pottery and other purposes as at Llwyneinion [SJ 288 4751 and Cae-llo [SJ 288 5431. They are not now utilised and since such clays and mudstones occurso commonly itis not feasibleto categorize them as a resource onthe map.

Some of the old clay pits have been or arebeing used as disposal sites for industrialor household waste.

40 Sandstone (S) Two types of sandstone areshown on Map 6. Thesehave been denoted as a) quartzitic sandstone and b) feldspathic sandstone. The Kinnerton Sandstoneis not included as a resource asoutcrop its is everywhere coveredby thick drift deposits.

The quartzitic sandstones are those within the Minera Formation and the Halkyn Formation (except the Gwespyr Sandstone). They are sandstoneswith a veryhigh quartz (silica) content and a low content of feldspar. In thelower part of the stratigraphical sequence these sandstones contain an appreciable proportion of calcium carbonate either as included fossil (shell) debris or as an intergranular cement. Normally the quartzitic sandstones are hard, butlocally and unpredictably the more carbonate-richtypes may weather to a disaggregated andvery pure silica sand.

The feldspathic sandstones includethe Gwespyr Sandstone at the oftop the Halkyn Formation and all the sandstones within the Bettisfield Formation (Productive Coal Measures). They contain a higher content of feldspar than the quartzitic types and may also have an appreciable content of iron oxides.In general they are softer thanquartzitic the sandstones andare moreliable to weather along bedding planesor otherplanes of weakness in the rock.

Both typesof sandstone have beenwidely exploited in the past. Thebasal sandstones of the Halkyn Formation have been worked silica-brickfor making and roadstonein extensive quarries northand south of Bwlchgwyn and atNew Brighton [SJ 275 5051, near Minera. Thesesandstones have also been quarried at Waun-y-Llyn [SJ 285 5791, and are being worked at present at Bryn y Gaer [SJ 315 5761, near Hope. TheCefn Rock, within the Bettisfield Formation, was widely used as a building stone, with many quarries aroundMoss [SJ 305 5351, Broughton and Gwersyllt. A sandstone within the Erbistock Formation, quarried at King’s Mills [SJ 350 4901 near Wrexham, was also used as a building stone; it is not included as a resource since its extentis not known. There are numeroussmall disused quarriesin all these sandstoneswhich provided local sources of stone for walling and building. The aggregate qualityof the sandstonesis not knownin detail though their mineral composition and weathering characteristics suggest that the feldspathic sandstones are likely to be of poor to moderate quality while the quartzitic sandstones may locally be of good quality. However, there are a number of factors which militate against their exploitation for this purpose. Firstly, many of the sandstone beds, particularly within the Bettisfield Formation, are relatively thin and often change rapidly in thickness, and thus are not amenable to large-scale quarrying operations; secondly, the weathering characteristicsof the sandstones arevery variable and unpredictable, especially the quartzitic type where a hard sandstone can change laterallyto a disaggregated sand within a few metres; thirdly, they are closely adjacent to very large resources of limestone of consistent and proven aggregate quality.

Sand and gravel resources (Thematic Map7, and Figure15)

This map shows the known distribution of sand and gravel within1 metre of the surface. Deposits less than 1 metre in thickness have been omitted.The deposits are largely classified as Glacial Sand

and Gravel on Map -2 Drift geology. Areas of potentially workable sand and gravel (see below)

41 are also shown. These have been divided into those areas where the deposit is believed to be continuous or almost continuous beneath overburden, and thosewhere it is thought to bediscon- tinuous. Sites of active and former workings for sand andgravel are shown. At present there are three active pits within the study area, at Fagl Lane [SJ 300 5861 operated by Welsh Aggregates Ltd, Borras Farm [SJ 355 5291 operated by Alfred McAlpine Quarry Products Ltd, and at Bank Farm [SJ 332 5731 operated by Astbury Quarries Ltd. Thewhole of the study area (except for a 2 km wide strip in the north-west ,north of grid line 50 and west of grid line 27) has beenpreviously assessed by the Industrial Minerals Assessment Unit (IMAU) of BGS and the results published (Dunkley, 1981; Ball, 1982). The boreholes drilled during that study areshown on the map,as are other boreholeswhich prove more than1 metre of sand and gravelwith an overburden ratioof less than 3: 1. In theirassessments a deposit statedto be ‘potentially workable’ mustsatisfy the following criteria: 1 The deposit should average at least 1 metre in thickness. 2 The ratio of overburden to sand and gravel should be no more than 3:l. 3 The proportionof fines (particlespassing an 0.625 mm BS sieve) should not exceed40 per cent. 4 The deposit should lie within 25 m of the surface.

For particle size analyses a grain size definition based on the geometric scale 1/16 mm, 1/4 mm, 1mm, 4 mm, 16 mm, and64 mm has been adopted. Theboundaries between fines(that is the clay and silt fractions) and sand, and between sand and gravel are placed at 1/16 mm and 4 mm respectively. During the present study minor amendments have been made to the boundariesof the sand andgravel depositson thepublished maps (Dunkley,1981; Ball, 1982). Additionally, the areas indicated on these maps as being ‘potentially workable’beneath superficial deposits have also been amended in the light of evidence from recent boreholes.No additional particle size analyses of deposits within the study area have been carriedout.

A number of factors affect the easeof working of the sand and gravel deposits; these include their topographic form and the degree of lateral variation within them. At one site (Fagl Lane) the deposit is being worked from below the water table, by dredging. In some areas, as between Wrexham and Gresford, the deposits have a marked moundy topographywith a number of enclosed hollows (kettle holes) which may be filled with peat and/or clay to a considerable depth. The variable thicknessof the sand and gravel in such moundy areas, togetherwith the infilled hollows, renders such deposits moredifficult to work on a large scale than flat-topped spreadsof sand and gravel as to theeast of Wrexham from Borras Airfield southwards. theAll sand andgravel deposits show rapid lateral changes of lithology. In particular the coarser gravels are often confined to irregular channels within a deposit of generally finer grade. The proportions of the various types of clast within the gravels also vary considerably. The main clasts are quartzites, limestones and Carboniferous sandstones,with smaller proportions of Lower Palaeozoic sandstones and siltstones and various igneous rocks. Coal forms a significant proportion of the clasts in some places. The proportion of Lower Palaeozoic clasts tends to increase towards the west and the igneous clasts towards the north, reflecting derivation from the Welsh and Irish Sea ice-sheets respectively. However, thesand and gravel deposits in general, and the Wrexham ‘delta-terrace’ (see Summary of Geology, AnnexA) in particular, werelargely deposited by water derived from both ice-sheets

43 and thevariation in clast content is minimal compared with that in the Welsh and Irish Sea derived tills.

Approximately 93 km2 of the study area has sand and gravel either at the surface or concealed beneathoverburden. The resultsobtained by theIMAU (Dunkley, 1981; Ball, 1982), with additional information obtained during the present study, indicate a total volume of deposit of about 9 x lo9 m3 (approximately 13.5 xlo9 tonnes). Of this at least 20% has already been sterilized by the urban areas of Wrexham, Rhostyllen, Gresford, Llay and Hope. The areas of greatest potential resourcelie along the Alyn valley from the northernmargin of the study area southwards to Hope and thence eastwards to Llay and Burton; east of Wrexham between Gresford and the Clywedog valley; and southof Wrexham between Marchwiel, Hafod and Bersham.

ENGINEERING GEOLOGY

Solid rock (Thematic Map8) and Drift(Superficial) deposits (Thematic Map 9)

The two engineering geology maps of the Wrexham area depict areas in which geotechnical conditions are broadly consistent. However, the maps are only summaries and cannot reflect detailed conditions onsite-specific a basis. The maps, and accompanyingthe engineeringgeological report (Waineet al.,1990) present information that provides abetter understandingof the general engineering geology of the areaso that potentialconstraints on development can identifiedbe and site-specific investigations better designed.

The boundariesbetween engineering geological units are based on thegeological maps produced during the study. Map 8 shows the engineering geology of the solid rocks and Map 9 that of the drift (superficial) deposits. The groupings of materials with similar geotechnical properties were derived, in part, from data extracted fromsite investigation reportsfor locations within the study area.

A detailed discussion of the engineering geology and descriptionsof thegeotechnical tests forwhich results were analysedare given in the accompanying report by Waine et al.,(1990).

Both maps should be examined to obtain an indicationof ground conditionsin a specific area. Map 4 will provide an approximate thicknessof any drift (superficial) deposits.It should be notedthat for areas shown as being drift-free, a mantle of weathered material and thin superficial deposits may be present. Itis also possible that undetected pocketsof thicker drift depositsmay occur. Map 9 also shows undifferentiated made ground andfill.

It is ernphasised that these maps present only a general guide to the ground conditions and that they should not be used, as a substitute for detailed site investigation, to ascertain the conditions prevailing at anyspecific site. However, the mapswill be useful as an aid to bettersite investigation

44 design, by indicating the conditions likely to be found a atgiven site. They arealso a valuable source of general information relevantto planning issues.

Maps 8 and 9 are based,respectively, on thesolid and driftgeology (Maps 1and 2). Therelationship between the engineering geology units and the solid rock formations and the drift (superficial) deposits are shown in the mapmargins and Tables3 and 4. These tables also show the engineering geological characteristics of each of the units. Simplified versions of the thematicmaps are given in Figures 16 and 17.

Summary values of geotechnical parameters for the units are presented in Tables 1 - 24 of the accompanying engineeringgeology report (Waineet al. , 1990). Where informationwas available, these descriptions include comments on engineering design considerations in relation to foundations,excavatability, suitability as fill materialand slope stability. These descriptions are summarised on the mapmargin.

Most of the units shown on the Engineering Geology of Solid Rocks map (Map 8) consist of 'engineering' rocks. However, because of weathering, some gradation between the statesof rock and soil (in engineering terms) occurs, particularlyin the case of the highly to completely weathered mudrocks of the Bettisfield and the RuabonMarl formationswhich weather toa softto stiff, silty clay.

SLOPE STEEPNESS

Slope steepnessis an important parameterin planning and development, providing a constraint for some land uses. There is a close relationship between slope steepness and factors such as slope stability, the design of gradients of roads andrailways, the use of agricultural machinery and certain types of construction plant, housing density and industrial development.

It is rarely possible to assign a precise slope steepness valueto what can or cannot be donesafely on a particular slope. Slopesin excess of 15" (1 in 4) may be stablein a coherent, massive limestone but unstablein a weak saturated clay. Most rocks andsoils have a threshold valueof slope steepness beyond which they become unstable. For any particular rockor soil type this valuewill depend not only on lithology but also on factors such as the presence or absence of discontinuities, the inclination of the bedding, the nature of the underlying and overlying strata, the degree of weathering, thehydrogeological conditions prevailing and vegetation cover.Therefore, the threshold slope steepness value for stability need not be the sameall slopes for of the samelithology and will vary according to the inter-relationshipof the controlling factors. However,given similar slope conditions, similar lithologies will tend to have consistent threshold slope values, but these may change with time as weathering of the slope progresses.

Table 3 Engineering geology units- solid geology

Engineering geology unit Geological unit Engineering geological characteristics

Limestones Minera Formation Variable mass strength dependent on frequencyof argillaceous bands anddiscontinuities. Dis- continuities enlarged dueto solution; rockhead uneven. Natural andartificial (mining) cavities Cefn Mawr Limestone may be present. Rippable where weathered but blasting necessaryin fresh rock. Suitable asfill after crushing. Design slopes should beno steeper than1:l where argillaceous bands present. Loggerheads Limestone

Strong sandstones Ruabon Marl Formation sandstones Strong when fresh, moderately strong when moderately weathered. Weather ultimately to a dense sand up to1.5 m thick. Excavation by blasting necessary for fresh rock but becomeripp- Bettisfield Formation sandstones able with increasing weathering.

Halkyn Formation sandstones

Interbedded mudrocks and Erbistock Formation Mudrocks are well jointed and weatherto asoft to hard silty clay to a depthof 1- 3 m below moderately strong sand- rockhead. Sandstones are up to 1.5 m thick, thinly bedded andfissile. Excavation by ripping, or stones Coed-yr-Allt Formation digging in weathered mudstones.

Weak sandstones Chester Pebble Beds Formation Coarse grained and cemented when fresh but weatherto asand which may be water-bearinggiv- ing rise to running conditions. Kinnerton Sandstone Formation

Mudrocks Ruabon Marl Formation mudstones and Weather completely to afirm to stiff silty clay, recorded up to4 m thick. Diggable and may be shales used as structuralfill. However, weathered material softens rapidly on exposure and wetting. Should be placed soon after excavation and subjectedto minimum constructiontraffic. Bettisfield Formation mudstones, shales and slitstones Halkyn Formation shales

Table 4 Engineering geology units - drift (superficial) deposits

~~ Engineeringgeology unitGeological deposit Engineering geological characteristics

Made Ground/Fill Made Ground/Fill Including tips,infilled excavations, landscaped ground. Materialvery variable (eg household rubbish,industrial waste, mining waste). Engineering geologycharacteristics are site specific dependent ontype of fill and method of emplacement. Detailed sitespecific investigation required.

Peat Peat Very soft; ranges from fibrous or decayed,to an amorphous organicsilty clay. Up to 1.2 m thick in alluvial deposits; also foundin upland areas. Highly compressible, consolidates rapidlywith large settlements. Removal and replacement may be required. Acidic.

Normally consolidated/me- Alluvial deposits Alluvial deposits consistof soft silty clays c. 1.5 m thick; medium denseclayey fine sand, c.3 m thick; dium dense heterogeneous medium dense silty sand and gravel from0.5 to 7 m thick. River Terrace deposits as latteralluvial de- soils River Terrace Deposits posit but with cobbles and boulders and thin bandsof firm silty clay and clayey silt up to 1.5 m thick. Clays compressible withlow bearing capacity, unsuitable asfill. Excavations need support and dewater- Head deposits ing. Head variable, typically sandy silty clays withgravel. Forms thin veneer onhillslopes, thickening to valley floors. May contain relict shear planes and be susceptibleto mass movement failures. Coarser material may contain perched groundwatertables. Careful investigationby pitting necessary.

Overconsolidated Till (Boulder Clay) Mainly silty sandy clays with gravel cobbles and occasional boulders. Bandsof sand and gravelwhich heterogeneous soils can be water bearing. Clays of low to intermediate compressibilitywith small settlements. Excavations stable in short term butmay deteriorate on wetting. Suitableas fill at low moisture content.

Overconsolidated cohesive Laminated Clay Till Laminated silty clay withbands of silt and fine sand (laminated clayey till) or well bedded clayeysilts laminated and medium (Glacial silt). Laminated clay may be desiccated to c.5 m from ground surface. Strength anisotropic. dense layered soils Glacial Silt Excavations in laminated clay stable in short term but deterioratewith wetting. Glacial silt may run in excavation andis unsuitable as structural fill.

Dense non-cohesivesoils Glacial Sands and Gravels Medium dense to dense sand and gravelwith cobbles recorded up to38 m thick. Medium dense sands and silty and clay sand bands present. Bearing capacity variable.

Landslip Variable composition; found on severallithologies on steepvalley sides (>15") and where toe undercut by streams. Careful investigation required to determine slopestability. Slope steepness has a variable rather than absolutewhich value, will alter with changes in the factors controlling slope equilibrium. These changes canbe brought about by natural processes or by the modification of the slope by engineering works. The controlling factors andthe effect of changes on them aresite-specific. Design judgements or slope stability assessment should be based on site measurements. No mapof slope steepnessis included with the report,but suchdata is held in digital form at the BGS Headquarters at Keyworth, Nottingham.This data was derived fromthe spacing of 5 m vertical interval contours on the 1:25 000 scale Ordnance Survey topographic maps. The values of slope steepness chosenas class boundaries aregenerally considered as bounding values for development, construction, agricultural and other land uses. The slopeclassification adopted in this study andits significance for land use are presentedin Table 5.

HYDROGEOLOGY (FIGURE 18)

There is a limited amountof hydrogeological data for thestudy area andconsequently it has been displayed on Figure 18 rather than as a 1:25000 scale Thematic Map. Thisfigure shows the main aquifers of the area, which are the Carboniferous Limestone, sandstones in the Millstone Grit (Halkyn Formation), sandstonesin the Productive Coal Measures (Bettisfield Formation) andthe Permo-Triassic sandstones (Kinnerton Sandstone and Chester Pebble Beds formations). It also shows the location of licensed wells, springs and adits,licensed landfill sites (Table7) and contours on the potentiometric surface of the Permo-Triassic sandstones. The drift deposits have been omitted fromthis map forclarity; they are shown on Map 2. Chemical analyses of the water from selected sources are shown in Table 8. Problems related to aggressive ground water and pollution of aquifers are considered in the section on Physical and Chemical Constraintsto Development.

The study area lies mainly within Hydrometric Area 67 (the Dee catchment), with the southern margin within Hydrometric Area54 (the Severn catchment). The managementof water resources lies in the handsof the National Rivers Authority (Welsh and Severn-Trent regions).

The main drainage of the area is by the River Dee and its tributaries, notably the Ceiriog. The southernmost parts are drained by the River Perry, a tributary of the Severn. The streams are moderately flashy with a base flow index varying from 0.49 on the Dee to 0.67 on the Perry. Extensive areas of the alluvial plain of the River Dee areliable to flooding or could be flooded if protectivelevees were breached or overtopped. These areas make up the ‘Dee floodplain safeguarding area’ (Figure 4) in which development is likely to be strongly influenced by the possibility of flooding.

The mean rainfall over the district is of the order of 700 mm/year over the lower ground to 900 mm/year over the higherground. The meanevapotranspiration is considered tobe about 450 mm/year. Infiltration is controlled largely by the thickness and permeability of the drift cover, varying on average between 50 and 250 mm/year.

50 Table 5 Slope steepness classes and examples of land use limitations (based on Small and Clarke, 1982)

~~ Angle Gradient Yo Housing and Construction Roadsand rail lines Agriculture Percent construction machinery

>15" About >27 Developing sites for Road buildingdiffi- Road and rail construction Slopes to steep for tractors. Mainlygiven over 1 in 4 housing and construc- cult. Absolute limits require increased site works to pasture. tion likely to require are approached for (e.g. cuttings) and costsas 11-15" 1 in 5 to 20-27 expensive precaution- most wheeled vehicles gradients increase. In gen- Absolute limits are approached for tractors. about ary workswith atten- eral, it is more economicto Ploughing generally not possible without con- 1 in 4 dant risk of triggering locate roads andrail lines to tour terraces. land instability in some take advantageof less steep 7-1 1" 1 in 8 to 13-20 locations. Problems for wheeled terrain. Problems for wheeled tractors and combine har- 1 in 5 vehicles. vesters.

3-7" About 1 613 Development for hous- Use of wheeled Problems for large-scale mechanicalagriculture. in 20 to ing and construction vehicles becomingdiffi- 1 in 8 likely to requirein- cult. creased costs forsite preparation.

<3" Less than <6 Suitable formost land uses whereother physical constraints are not present and where norisk of flooding. Poor drainage onflat clayey about 1 in ground and floodrisk present on flat,low lyingareas. 20

Table 6 Licensed abstraction from shafts, boreholes and springs inthe study area (m3/day)

CarboniferousBettisfieldPermo-TriassicHalkyn Drift Limestone Formation Formation sandstonesFormationFormationLimestone

Public supply Public - - 32617 1091 - and industrial Agricultural 54 19 37 311 125 and domestic

TOTALS 54 19 32654 1402 125

Total licensed abstraction fordistrict is 34 254 m3/day

A consideration of the hydrogeology of the formations presentwithin the district follows. Carboniferous Limestone

The limestone outcropoccupies only limited areaswithin the study area.While it has some potential for groundwater supply from boreholes, drilling for water is highly speculative as the limestones have minimal primary porosities and permeabilities, and groundwater movement is restricted to fissures enlarged by solution. There are norecords of shafts or boreholesconstructed specifically for water supplyin this district. However, two spring sourcesare licensed for a totaltake of 54 cubic metres perday (m3 /d) foragricultural and domestic purposes.

Halkyn Formation (Millstone Grit)

This formation comprises a multi-layered aquifer with groundwater obtainable from shafts and boreholes intersecting sandstone and grit beds; the intervening mudstones and shales are rarely permeable. Although the arenaceous beds offer some intergranular storage, groundwaterflow is generally through jointsand fissures. Shallow shafts and boreholesmay yield small supplies of the order of a few cubic metres per day, sufficient for domestic and small agricultural demands. Licensed sources comprisethree springs andone shallow shaft with a totallicensed take of 19 ms/d. Groundwater quality is likely to bereasonably good with atotal hardness of less than 300 milligrammes per litre (mg/l) as CaC03. Themain problem is generally with iron which is locally present in concentrations of more than 1.0mg/l; such values may impart a taste to the water and also cause stainingof laundry and bathroom fittings.

Bettisfield Formation (Productive Coal Measures)

This formation also comprises a multi-layered aquiferwith groundwater being foundin the sandy beds. For the few records that are available for this district, it would appear that a borehole of 300 mm diameter penetrating 30 metresof saturated rock would on averageyield between 60 and 70 m3/d for a drawdownof 10 metres. Therewould be approximately a 20% chancethat theyield would be less than 20 ms/d for the samedrawdown. However, abandoned coalworkings do provide a large although localised storage; a borehole (25 SE/43) at the Brymbo steel works [2921 53541

53 Table 7 Landfill waste disposal sites in the study area

Site National Grid Status reference

Willow Hope SJ 309 579 Closed before 1977 Fag1 Lane SJ 303 586 Closed before 1977 Tatham Road SJ 309 451 In use or closed after 1977 Astbury’s Quarry SJ 330 574 In use or closed after 1977 Gwersyllt SJ 318 543 In use or closed after 1977 Bwlchgwyn SJ 267 535 In use or closed after 1977 Brymbo SJ 291 531 In use or closed after 1977 Llanypwll SJ 365 518 In use or closed after 1977

Types: 1. householdand commercial 2. industrial3. mixed penetrated such workings and yielded 1700 m3/dfor a drawdown of 27 metres. The old Park Day Level (25 SE/1446) [2712 51601 and Speedwell Shaft(25 SE/1428) [2682 51321are eachlicensed for a take of 9800 m3/d. The total licensed abstraction from the Bettisfield Formation is 32 654 m3/d. Of this, only 37 m3/d is taken from shallow shafts and springs for agricultural and domestic use, while the rest is used for public supply and for industrial purposes,mainly from old coalworkings. In away, it is remarkable thata formation thatis generally consideredto be buta poor aquifer can supply such largeyields of groundwater, albeit from large and extensive systemsof shafts andlevels.

Under naturalconditions, groundwater in the Bettisfield Formation may be of fair quality, with a total hardness of less than 300 mg/l, and a chloride ion concentrationof less than 30 mg/l (as Cl). Iron, in shallow shafts and boreholes, may be present in low concentrations, but does tend to increase with depth. Waterdrawn from oldcoal workings may be significantly more mineralised. At the Brymbo site, the total hardnesswas over 440 mg/l, about half of which was non-carbonate hardness, while the chloride ion concentration was more than 40 mg/l. The concentration of iron exceeded 4.0mg/l. Elsewhere, undersimilar conditions, iron concentrationsof more than 10.0mg/l and manganese concentrationsof more than7 mg/l have been recorded.

Permo-Triassic sandstones

This is the only aquifer, in the generally recognised sense, within the study area. The sandstones are generally weakly cemented with a porosity ranging from 20% to 30%.Intrinsic permeabilities vary, but eventhe higher values are significantly less than thehydraulic conductivities encountered in the field. Thisis because groundwater movementis controlled by fissures that provide by far the greater partof the permeability and are presenteven to depthsof 100 metres. Transmissivities and borehole yields may vary considerably depending uponthe degreeof development of the fissures, but it is unusual for a “dry” boreholeto be drilled. Waterin the fissures is supplemented by inflow from poreswithin the rock matrix. A studyof borehole performancein the generalregion, including Cheshire, suggests that theaverage yield of a boreholeof 300 mm diameter penetrating30 metres of saturated rock would be of the orderof 700 m3/d for a drawdownof 10 metres. Therewould be

54 Table 8 Chemical analysis of water sources

Accession National Grid Location TDS TH CH NaCa Mg K HCO3 Number (SJ) Reference

25 NE1564 2910 5900 Pen-y-wern 250 214 25 SEl2B 2923 5214 Vron Colliery 25 SEI43 2921 5354 Brymbo Steelworks447 1032 227 91 53.5 495 34 NW11 3146 4817 Bersham Colliexy 34 NWl384 3308 4628 Old Sontley 320 334 168 67.4 37 11.8110 3.2 35 NWl26 3449 5739 Fields Farm, Burton 12.8 297 35 NWl324 3126 5543 Cefn-y-bedd 35 NWl386 3236 5859 Shordley Hall, Burton 1646.28 10112 712.4 36.5 71 35 NE114 3536 5867 Golden Grove, Burton260 322 260 2.4 35 SWl28 3356 5005 Border Brewery, 333 474 203 Wrexham 35 SEI2 3793 5483 Parkside, Allington 35 SEI81 3624 5177 Llan-y-pwll 395 634 208 2.77 13929.2 11.4 208

Accession c1 so4 No3 Fe BalanceMn PH Aquifer Date Type Number (SJ) - ______25 NE1564 21 37 7.5 Dec 72 Drift Spring 25 SEl2B 35 91 7.5 6.4 May 77 Coal Measures WelVAdit 25 SEI43 42.4 4.4 0.09 0.14 Aug 80 Coal Measures WelVAdit 34 NWl1 185 8.3 Dec 76 Coal Measures WeWAdit 34 NWl384 31.55.09 44.9 0.06 7.1 - 47% JuI 80 Drift WelVAdit 35 NWl26 56 0.28 7.3 Dec 78 Drift WeWAdit 35 NWl324 26 46 3.1 6.3 Sep 77 Coal Measures WelVAdit 35 NWl386 17.1 2960.09 2.16 0.086 7.16 -15% Jan 84 Drift WelVAdit 35 NE114 16 0.27 0.24 7.4 Dec 66 Permo-Trias WelVAdit 35 SWl28 60 117 7.46 Apr 48 Coal Measures WelVAdit 35 SEI2 27 7.6 May 73 Drift WelVAdit 35 SEI81<0.02 57.4 74.7 <0.01 7.51 - 29% Apr 84 Drift WelVAdit

TDS = Total Dissolved Solids. TH = Total Hardness, CH = Carbonate Hardness (both as CaC03). Units in mg/l (except for pH). approximately a 20% chance that the yield would be less than 230 m3/d for the samedrawdown. However, a borehole(35 SE/53a) of 300 mm diameter atMessrs Kelloggs in Wrexham [3893 50681 yielded more than1000 m3/d for about34 metres of drawdown even though was it lined out towithin 10 metres of the bottom. It is, however, uncertain if this borehole is in the Permo-Triassic sandstones or the Erbistock Formation. The totallicensed abstraction from the Permo-Triassic sandstones is 1402 m3/d. Of this, 311 m3/d are taken from 22 boreholes and shallow shafts for agricultural and domestic use, while the remainder is for a single industrial source. There are no public supply sources locatedwithin this aquifer in the Wrexhamdistrict.

The quality of groundwater from the Permo-Triassic sandstonesis generally very good, with a total hardness of 250 to 350 mg/l. The chloride ion concentration is usually less than 30 mg/l. While analytical data are rathersparse within the district, the concentrationsof iron and manganesemay approach 0.3 mg/l.

Drift deposits

Groundwater has also been exploited from the more permeableof the drift deposits, notably the sands and gravels. These are most widespread around Wrexham and Gresford; they also extend eastwards for somedistance under impermeable cover andgive rise to artesian conditions on the Wrexham Industrial Estate and elsewhere. In the coarser gravels, shallow wells can sometimes provide good yields. A borehole (35 SW/15) of 150 mm diameter and 10.6 metres depth at Wrexham [326 5071 yielded nearly 550 m’/d for a drawdownof 1.2 metres;in this particular case, much of the yield may have in fact come from induced recharge fromthe adjacent river.In general, yields are very variable, the mean being of the orderof 20 m3/d, and 20% being less than 5 m3/d. Eighteen sources,mainly shallow shafts and boreholes, butwith a few springs, have beenlicensed for a total takeof 125 m 3/d; all are fordomestic and agricultural use.Under naturalconditions, the groundwater is characterised by a low total hardness (less than 200 mg/l) and often by high concentrations of iron and manganese. In agricultural districts, high concentrations of nitrate are commonly found.

GEOLOGICAL FACTORSFOR CONSIDERATION IN LAND-USE PLANNING

Introduction

Interpretation of geological information may be required if planning policies are to beapplied in the most cost-effective manner. The main areas in which geological conditions affect land-use planning are:

Mineral resource potential (Thematic Maps5,6,7, 10 and Figure 19)

Physical and chemical constraints to development (Thematic Maps8,9,10 and Figure 20)

56 Mineral resource potential

The thematicmaps provide dataa sourcewith regard tomineral resources and their working. They delimit the various finite bedrock and drift resources (Maps6 and 7) and provide information on the quality of the deposits. Such information may assist in avoiding the sterilisation of valuable resources by development, including the effective siting of plant. A summaryof the main mineral resources, except coal, is shown on Figure 19. Waterresources are considered in the section on Hydrogeology and in Figure 18.

Sand and gravel Significant sand andgravel deposits occurin many partsof the area,particularly around Wrexham and northwards to Marford and Hope (Map7). Currently three sites are exploited for aggregate production, and deposits have been workedat othersites in the past.

Resources at surface and beneath limited overburden cover approximately93 km2. Deposits are relatively thin and rarely exceed 30 m in thickness; consequently despite the large extent of the resource area its maximum theoretical volume is limited (about 13.5 x lo9 tonnes). Extraction in commercially viable quantities necessitates pits of large areal dimensions, which are relatively ephemeral compared to those associated with bedrock quarrying. The deposits tend to occur in areas of higher grades of agricultural land, or in close proximity to built-up areas, or in areas identified as being of high amenity value. Already some20% of the theoreticalresource has been sterilised by urban development. It should be noted that sand andgravel deposits often occur as moundy topographic features and their removal alters the landscape in a way which is hard to restore to its original form. These factors raise problemsfor thedefinition of future resources.

Potential resources, including concealed deposits, were identified by the Industrial Minerals Assessment Unit of the British Geological Survey (Dunkley,1981; Ball, 1982). Areas of greatest interest appear tolie along the Alyn valley from the northernmargin of the study area, southwards to Hope and thence eastwards to Llay and Burton, east of Wrexham between Gresford and the Clywedog valley, and south of Wrexham between Marchwiel, Hafod and Bersham. The deposits are very heterogeneous andvary greatly in thickness. Consequently the qualityof the resource can only be provedby site-specific investigations.

Limestone Only a smallarea (5 km2) of limestone outcrop is present within the study area,with the theoretical amount of resource totalling somelo9 tonnes. However, this forms partof the extensive Carbon- iferous Limestone outcrop in north and east Clwyd which providesa valuable resource for aggregate, cement, industrial and agricultural purposes. The outcropwithin the study area has been divided into three formationswith distinct resource characteristics (e.g. purity, limestoneto shale ratio (Table2)). Of these, theLoggerheads Limestoneis of high purity with minimal shale content. The limestones ofthe Cefn Mawr Limestone and Minera Formation arealso of high purity, but are interbedded with shales and, in the Minera Formation, sandstones also. Aggregate is produced from onesite in the study area andhas been extracted from a numberof other sites in the past (Map

6). Apart fromits resource potential, limestoneis also a significant source of groundwater outside the study area and,as such, requires protection from inappropriate developmentor contamination.

Underground mining of coal by shallow or deep methods(pillar and stallor longwall) and, more recently, opencast mining,has been a major industryin much of the central partof the areain past years (Map 5). The last deep mine closed in 1987; at present thereis no deepmining and only one opencast site is working. Economic considerations suggest that future extraction is likely to be possible from limited opencast sites only.The generallocation of potential opencast coal resources is coincident with the areain which there may have been mining at shallow depth; on Map10 this is defined as the outcropof the Bettisfield Formation (Productive Coal Measures) wherethe drift cover is less than 30 m thick. Within that area workableresources are likely to be at or above the level of the lowest normally worked seam,the Llwyneinion Half-Yard. The potential for opencast extraction dependson a numberof factors apart from environmentalconsiderations. These include the number,thickness and qualityof individual seamsin any given area, thethickness of overburden and the extent whichto the seams have already been removedby underground mining or disturbed by faulting.

Sandstone Sandstone has been workedextensively in the past for building and walling stone and forrefractory materials. At present, only one quarry is working, at Bryn y Gaer [SJ 315 5761, near Hope. Sandstones of differing resource charactersare identified in the report(see Map 6). Theseresources are scattered widely across the area and most appear to beof little economic importance. Thus, resource sterilisation of sandstone is unlikely to bea major planning consideration.

Brickclay Brickclay (for tile making)is extracted from onesite in the RuabonMarl Formation. Although the outcrop of this formation covers an extensive areais almostit entirely coveredby thick superficial deposits. Consequently, apart from the small area indicated on Map 6, sterilisationof this resource is unlikely to be a problem.Fireclays were worked in open pits or mines at a number of horizons within the Productive Coal Measures, usually in association with coal; however, the deposits are generally thin and notof great economic importance.

Lead, zinc and copper ores Considerable mining of lead and zinc, and a littlecopper, took place largely in the second half of the nineteenthcentury but noneis now extant. Itwas restricted to the area south west and of Minera. Although resources still exist, particularly at depth, their extractionis uneconomic in present or foreseeable terms due to drainage problems and the unpredictable nature and quality of the resource. This aspectis therefore not considered significanta concern for future planning. Aswith coal mining, it is the results of mining which provide the most important considerations for development and land use.

60 Physical and chemicalconstraints to development

The main physical constraints which may affect land-use planning decisions are shown on Map10 and in a simplified version in Figure 20. In addition it gives a summary of other factorswhich may also affect planning decisions, and referenceis made to the appropriate Thematic Map.

Slope stability Natural slope instability posesfew problems to planning and development in the study area, but modifications by engineering workswhich adversely alterthe conditions controlling slope equilibrium may trigger mass movements locally, for example,in excavations and cuttings. Such failures are most likely to occur in the drift (superficial) deposits and in the weathered mudrocks. Steep slopes are notnecessarily inherently unstable (see sectionon Slopesteepness) ,but theymay impose cost constraints on site development.The likely constraints are listed inTable 5.

Landslips (Map 10) have been observedon numerousrock types; they havein common a location on steepvalley slopes (in excess of 15"), which are at presentoccupied by a river or stream. The largest landslips have occurred in mudrocks within the Bettisfield Formation, e.g. at [289 5481, often originating from just beneath a sandstone/mudrock junction. The largest slip which is not within mudrocks [252 4941 occurs within limestone overlainby sandstone andmay have been caused by failure along a bedding planesince the angle or inclination is sub-parallel to that of the slope. Smaller slips have been observed within drift deposits which are being actively eroded by watercourses.

It is advisable to suspect the presence of landslips, where the above-mentionedfactors are found in combination, even thoughlandslipping at a selected sitemay not be indicatedon published maps. Site investigations should determine the presence or absence of landslipping, and obtain the necessary soil strength and other parameters to enablea stability analysis to becarried out for the proposed slope profile after completionof construction work. Trial pitting and carefullogging and sampling may be the most appropriate investigation method, depending upon the depth of slip planes.

Head deposits (variable deposits,mainly of silt and clay, formed underperiglacial conditions; also recent hillwash) have not been mapped separately andare notindicated on themaps. Where they are greater than about1 m in thickness they have been includedwith till. Such deposits needto be treated with caution as they may contain relict shear planes. Even siteson low angle slopes can be affected. Surface bulges may indicate recent ground movements which can be reactivated by construction works. It is also important to note that headdeposits of less than 1 m thickness are widespread. Trial pit excavation and detailed logging during excavation is recommended. Shear planes areoften exposedduring excavation and can beidentified by theirpolished, often slickensided surfaces. Logging the sides of an excavation pit alone will not necessarily reveal the shear planes.

61 Foundation conditions Compressible Soils (soft ground) The compressibility of a soil (see Glossary for an engineering definition of ‘soil’) may be defined asthe volumetric strain per unit pressure increase. The overall effect of imposing a foundation loadon a normally consolidated depositis to compress the deposit. The vertical component causes-a vertical movementof the foundation due toelastic bending and reorientation of the soil particles. Excess hydrostatic pressure is gradually dissipatedby expulsion of water from thesoil voids, giving rise to time dependent volume change.The vertical component of the changeis called the consolidation settlement.

Soils which experience relatively large amounts of settlement, or differential settlement are problematic, and can be identifiedby their geotechnicalparameter values obtained from laboratory tests. These soils are predominantlyvery soft to soft,normally consolidated clays, which are often organic, and peats. In the study area,such deposits are found in the valleys of the water courses (alluvium) and within approximately circular, marshy depressions knownas kettle holes (Maps 2 and 10); hill peat is present as a thin cover on partsof the eastern slopeof Esclusham Mountain. Except on thehills the two lithologiesare often interbedded,with peat forming lensesrather than discrete beds.In the kettleholes, which are prominentwithin the glacial sands andgravels between Wrexham and Gresford, soft deposits have been recorded as being between10 and 20 m thick and are best avoided forbuilding purposes if possible. Inthe river valleys and on thehills the thickness probably does not exceed2.5 m, andremoval and replacementwith compacted hardcore has been recommended. Where it is thicker, this may be uneconomic, and ground treatment is required before shallow foundations can be utilised, for example by reinforcing the alluvium with stone columns by the vibroreplacement technique. Alternatively, end bearing piled foundations may be employed to transfer heavier loads to more competent strata beneath the soft deposits, where present. Caution should be exercised where the clay is soft to firm as it is often desiccated at the surface, becoming softerwith depth.

Running Conditions This term refers to predominantly granular soils which are saturated with water and which may flow into excavations and boreholes due to therelease of pressure. Under high hydrostatic pressure the material can rapidly enter the excavations as a fluidised flow. Trial pits become unstable and collapse, and borehole drilling is hampered. Running conditions have been reportedwhen excavating within alluvial sands andgravels adjacent to watercourses, glacial silts, sands andgravels within till and weathered Kinnerton Sandstone Formation sandstoneclose to theriver Dee near Bangor-is-y-coed. Sheetpile cutoffs, shoring andwell point dewatering have been employed to maintain stable excavations.

Aggressive Groundwater When theconcentration of sulphates presentin groundwater andin soil exceeds a threshold value,any buried concrete structureswill be attacked, corroded andweakened unless special cement typesare used. Similarly where acidicgroundwater andsoils are found,buried structures andservices can be corroded. As the concentrationof corrosive substances increasesso does thenecessity of utilising Sulphate Resisting Portland Cement for foundations.

The highest concentrations of SO3 have been obtained from sites in the vicinity of colliery spoil heaps in the Gresford area.

62 Acidic groundwater is associated with peat and buried structures within peaty soils should be designed to withstand corrosion.

Cavities Certain strata in the area contain natural andartificial cavities which may be near, or at, surface. Firstly, natural solutioncavities are presentin the limestones and may be infilled by drift deposits washed into them. Secondly, there are disused mineral workings within veins in the limestone and Halkyn Formation sandstones around Minera; also, the Bettisfield Formation strata have been mined €or coal, ironstone and fireclay, and disused shafts, adits and workingsold may lie close to the surface. Map 10 shows the areas in which shallow old workings for coal are most likely to exist. They cover the outcropof the Bettisfield Formation, but exclude those areas with a drift thicknessexceeding 30 m. These hiddencavities and soft zoneswill reduce thebearing capacity of overlying strata andmust be identified duringthe investigation. The position of old workingsis often shown on Ordnance Survey maps. Known shafts and aditsare shown on Map5. If they are shown to underlie the site, specialist advice should be sought. Even so, not all workings can be detected in this manner, and Bell (1988) provides useful information with respect to means of locating abandoned shafts. Piggott and Eynon(1978) and Garrard andTaylor (1988) give valuable information on ground movementsresulting from shallow mine workings. Oldworkings can also act as reservoirs formethane given off from coal seams (seebelow in section on Backfill etc).

Swelling Ground None of the geotechnical reports studied whilst compiling this report referred to, or tested for,swelling potential. Swelling takes place in some mudstone strataas a responseto an increase in moisture content, and can give rise to relatively large volume changes which may damage foundations not designed allowto for swelling. Conversely desiccationallows the clays to shrink leading to excessive settlement. In view of this, swelling potential of weathered Bettisfield Formation mudstoneswas investigated andis documented by Entwisle (1989). For therocks tested, swelling potential was found to below. However, theresults were obtained from asmall number of samples from one locality. It is suggested that whereplasticity levels are high (Liquid Limit 60, Plastic Limit 35) as found in rocks of weathering grades I11 and VI in the Deeside area,swelling characteristics are measured.

Backfill, Made groundand Landscaped ground These three categories (shown on Map 10) all result from human disturbance or alteration of the natural ground surface. They havein common a variable composition, rate of consolidation and thickness; consequently their engineering propertiesare also variable (seeMap 9 and the report on Engineering Geology (Waine et al., 1990). Additionally there is the possibility of hazardous or methane-generating components being present in backfill or made ground, and spontaneous combustion may occur in colliery waste under certain conditions (see below).

Methane This explosive gas can be generated by breakdown of domestic refuse within landfill sites. In some modern landfills, the gas has been utilised as a source of energy and the landfill designed accordingly. However,there havebeen reported cases, outside the study area, of explosions following the migration of gas from badly designed dumps, often dating from a time when the hazard was not appreciated. The Local Water Authorities have information regarding the types of waste buried in landfill sites which began operation since 1974 when the governing

63 legislation was introduced. Pits and quarriesbackfilled before thenmay contain materialwhich can break down torelease methane.

Methane gas canalso be given off from coal seams. Evenif proposed worksdo not intercept seams, old workings can serve as zones of gas accumulation. During an investigation near Gresford, methane was detected, anda coal mine shaft suspected, though no recordof its existence couldbe found. Methane is soluble, and care must be taken when tunnelling within or adjacent to coal bearing strata, as the gas can exsolve from groundwater and accumulate,in addition to migrating through discontinuities in its gaseousstate.

Spontaneous combustion Colliery waste tips are vulnerable to spontaneous combustion during excavation, handling andplacing. The main conditions necessary for it to takeplace are: a sufficiently high combustible carbon contentto provide a fuel (generally over25%) a sufficiently high iron pyrites content, oxidation of which generates the heat for spontaneous combustion a sufficient availability of oxygen to sustain the combustion

For thetips near Gresford, the amountof combustible carbonwas reported to besufficient to cause fire. However, the iron pyrites content has been considered too low to generate spontaneous combustion, though this could not be totally ruled out.

When engineering works are carried out within colliery waste tips, compaction of embankments and shallow slopes are necessary to reduce the amountof available oxygen so preventing combustion. The slopefaces should be protected with a thicker than averagecover of clay or topsoil.

Groundwater Groundwater is obtained (Figure 18) from the Carboniferous Limestone, sandstones within the Halkyn and Bettisfield formations, the Permo-Triassic sandstones (Kinnerton Sandstone and Chester Pebble Beds formations) and from drift deposits (see section on Hydrogeology).

Superficial (drift) aquifersare very vulnerable to pollution from surface sources,due largely to the closeness of the water table to the ground surface and to the generally high permeability of the deposits.

Groundwater from bedrock aquifers is generally less vulnerable to pollution than that from superficial aquifers becauseof the filtering and attenuating effectsof the unsaturated zone above the saturated aquifer. Consequently,it usually receives littletreatment before being pumped into supply, and it is important that aquifers are protected from potential pollutants. mostThe vulnerable aquifers are those where water passes rapidly from the surface into the saturated zone.Such aquifers include the Carboniferous Limestone (Map 1) where most of the groundwater flow is through joints widenedby solution, and sandstonein the Bettisfield Formationclose to old workings (Maps 1 and 5).

64 There are two main types of potential pollutant sources, point and diffuse. Point sources include landfill and other waste disposal sites (such as sewage treatment works), and storage tanks for silage, fuels, industrial solvents andother chemicals. There areseveral sitesthat have acceptedin the past or arecurrently accepting both household and industrialwaste (Table 7). In general, these pose few potential problems for groundwater, either because they are located upon the outcropof strata such as the Bettisfield Formation anywhere lateral spreadof leachate is limited by the faulted and multi-layered aspectsof the strata, orbecause the bedrock is covered by extensive deposits of relatively impermeable drift. Where the aquifer outcrop is drift-free, as is the case with the Carboniferous Limestone and some of the sandstones in the Halkyn and Bettisfield formations (Maps 1,2and 4), storage tanks can represent a seriousrisk to groundwater quality,particularly so where they arepoorly constructed or in a bad state of repair.

Diffuse sourcesof pollution are largely represented by nitrates, appliedto the groundas fertiliser, and biocides; both are widely used in agriculture. However, the risks in this district are generally low in respect of the two major water-yielding formations, the Bettisfield Formation and the Permo-Triassic sandstones, due to theextensive coverof relatively impermeable drift.

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66 EARP, J R.1958. Mineral veinsof the Minera- Maeshafn districtof North Wales. Bulletin of the Geological Surveyof Great Britain,No. 14,44-69. EARP, J Rand TAYLOR, BJ. 1986. Geology of the country around Chester and Winsford. Memoir of the British Geological Survey,Sheet 109 (England and Wales). EMBLETON, C. 1970. North-eastern Wales. 59-82 in The Glaciationsof Wales and Adjoining Regions. LEWIS, C A (Editor). (London: Longman.) ENTWISLE, D C.1989. The swelling characteristics of weathered Coal Measures mudstones from North Waleswith reference to theswelling potential of weathered Coal Measures mudstones in general. British Geological Survey Technical Report,WN8918. FITCHES, W R and CAMPBELL,S D G. 1987. Tectonic evolution of the Bala Lineamentin the Welsh Basin. Geological Journal,vol. 22 (Spring Thematic Issue),131-153. GARRARD, C F Gand TAYLOR, R K.1988. Collapse mechanisms of shallow coal-mine workings from field measurements.181-192 in Engineering Geology of Underground Movements: 23rd Annual Conference of the Engineering Groupof the Geological Society. BELL, F G., CULSHAW, MG., CRIPPS, J C., LOVELL, M A.(editors) London: Geological Society Engineering Group Special Publication,No. 5. GEOLOGICAL SURVEY OF GREAT BRITAIN.1920a. Refractory materials: Ganister and silica-rock - Sand for open-hearth steel furnaces- Dolomite. Special Reporton the Mineral Resources of Great Britain, Memoirof the Geological Surveyof Great Britain,Vol. 6. GEOLOGICAL SURVEY OF GREAT BRITAIN.1920b. Refractory materials: Fireclays. Special Reporton the Mineral Resourcesof Great Britain, Memoirof the Geological Surveyof Great Britain, Vol. 14. GEORGE, TN. 1974. Lower Carboniferous rocksin Wales. 85-115 in The Upper Palaeozoic andpost-Palaeozoic rocksof Wales. OWEN, T R (Editor). (Cardiff: Universityof Wales Press.) HARRISON, DJ, MURRAY , D W and WILD, J B L.1983. Reconnaissance Survey of Carboniferous Limestone resourcesin Wales. (Keyworth, Nottingham : Institute of Geological Sciences. HEAD, K H.1982. Manual of Soil laboratory testing.(London : Pentech Press.) HIND, Wand STOBBS,J T. 1906. The Carboniferoussuccession below the Coal Measures in north Staffordshire, Denbighshire and Flintshire.Geological Magazine,Vol. 43,387-400, 445-459,496-507. JACOBS, C AJ. 1982. Mineral Working in Clwyd. Clwyd County Minerals Plan, Final Draft Written Statement,Clwyd County Council. JONES, R C B and LLOYD,W. 1942. The stratigraphy of the Millstone Gritof Flintshire. Journal of the Manchester Geologists Association,Vol. 1,247-262. LAMBE, T W and WHITMAN, R V.1979. Soil Mechanics (S.I. Version).(New York : Wiley) LOUDON, T V, CLIFTON, A W, HOLMES,K A, LAXTON, J andL MENNIM, K C. 1991. Wrexham applied geological mapping project: computing techniques. British Geological Survey Technical Report,WO/91/1.

67 MAGRAW, D and CALVER, M A. 1960. Faunal marker horizonsin the Middle Coal Measures of the NorthWales coalfield.Proceedings of the Yorkshire Geological Society,Vol. 32,333-352. MORTON, G H. 1870. On theMountain Limestoneof Flintshire and partof Denbighshire. Geological Magazine,Vol. 7,526-527.

MORTON, G H. 1876-1879. The Carboniferous Limestone andMillstone Grit of North Wales - country between Llanymynech and Minera.Proceedings of the Liverpool Geological Society, Vol. 3, 152-205,299-325,371-428. NEAVERSON, E. 1946. The Carboniferous Limestoneof North Wales: Conditions of deposition and interpretation of its history.Proceedings of the Liverpool Geological Society, Vol. 19,113. PEAKE, D S. 1961. Glacial changes in the Alyn river system and their significance in the glaciology of the NorthWelsh border. Quarterly Journal of the Geological Societyof London, Vol. 117,335-366. PIGGOTT, R J andEYNON, P. 1978. Ground movementsarising from thepresence of shallow abandoned mine workings. 749-780 in Large ground movements and structures: proceedings of the Conferenceheld at theUniversity of Wales Instituteof Science and Technology,Cardiff 1977. GEDDES, J D (editor). (London : Pentech Press) POOLE, E G and WHITEMAN,A J. 1961. The glacial drifts of the southern partof the Shropshire-Cheshire basin.Quarterly Journal of the Geological Societyof London, Vol. 117, 91-130. POOLE, E G and WHITEMAN,A J. 1966. Geology of the country aroundNantwich and Whitchurch. Memoir of the British Geological Survey,Sheet 122 (England and Wales). RAMSBOTTOM, W H C. 1973. Transgressions and regressionsin the Dinantian : a new synthesis of British Dinantian stratigraphy.Proceedings of the Yorkshire Geological Society, Vol. 39,567-607. RAMSBOTTOM, W H C. 1974. The Namurianof North Wales. 161-167 in The Upper Palaeozoic andpost-Palaeozoic rocksof Wales. OWEN, T R (Editor). (Cardiff: University of Wales Press.) RAMSBOTTOM, W H C. 1977. Major cycles of transgression and regression (mesothems)in the Namurian.Proceedings of the Yorkshire Geological Society,Vol. 41 , 261-291. RAMSBOTTOM, W H C, CALVER,M A, EAGAR,R M C, HODSON, F, HOLLIDAY,D W, STUBBLEFIELD,C J, and WILSON, RB. 1978. A correlation of Silesian Rocks in the British Isles. Geological Society of London, Special Report,No. 10. SMALL, R.J. and CLARK, M.J.1982. Slopes and Weathering. (Cambridge: Cambridge University Press.) SMITH, B.1921. Lead and Zinc oresin the Carboniferous rocksof North Wales. Special Report on the Mineral Resourcesof Great Britain, Memoirof the Geological Surveyof Great Britain, Vol. 19.

68 SOMERVILLE, I D. 1979a. Minor sedimentarycyclicity in late Asbian (UpperDl) limestones in the Llangollen district of North Wales. Proceedings of the Yorkshire Geological Society,Vol. 42. 317-341. SOMERVILLE, I D. 1979b. A sedimentary cyclicity in early Asbian (lowerDl) limestones in the Llangollen district of North Wales. Proceedings of the Yorkshire Geological Society,Vol. 42, 397-404. STRAHAN, A.1890. The geology of the neighbourhoods of Flint, Mold and Ruthin. Memoir of the Geological Surveyof Great Britain,Quarter-sheet 79 SE (England and Wales). THOMAS, G S P. 1985. The lateDevensian glaciation alongthe borderof northeast Wales. Geological Journal,Vol. 20,319-340. THOMAS, G S P. 1989. The lateDevensian glaciation along the western margin of the Cheshire-Shropshire lowland. Journal of Quaternary Science, Vol. 4,167-181. WAINE, P J, CULSHAW, M G. and HALLAM,J R. 1990. Engineering geology of the Wrexham area.British Geological Survey Technical Report,WN/90/10. WEDD, C B andKING, W B R. 1924. The geology of the country around Flint, Hawarden and Caergwrle. Memoir of the Geological Surveyof Great Britain,Sheet 108 (England and Wales). WEDD, C B, SMITH,B and WILLS, LJ. 1927. The geology of the country around Wrexham- Part I: Lower Palaeozoic and Lower Carboniferous Rocks.Memoir of the Geological Surveyof Great Britain,Sheet 121 (England and Wales). WEDD, C B, SMITH,B and WILLS, LJ. 1928. The geology of the country around Wrexham- Part 11: Coal Measures and Newer Formations.Memoir of the Geological Surveyof Great Britain, Sheet 121 (England and Wales). WILSON, A C, MATHERS,S J and CANNEL,B. 1982. The Middle Sands, a prograding sandur succession;its significance in the glacial evolution of the Wrexham-Shrewsbury region. Report of the Instituteof Geological Sciences,No. 8211. WOOD, A.1936. Goniatite zones of the Millstone Grit Seriesin North Wales. Proceedings of the Liverpool Geological Society,Vol. 17,lO-28. WOOD, A.1937. The non-marine lamellibranchs of the NorthWales coalfield. Quarterly Journal of the Geological Societyof London, Vol. 93,l-22. WREXHAM MAELOR BOROUGH COUNCIL.1989. Wrexham Maelor Local Plan. WREXHAM MAELOR BOROUGH COUNCIL.1990. Wrexham Maelor Local Plan; Draft Review.

69 ANNEX A SUMMARY OF GEOLOGY

The generalised geological sequence of the areais shown in Figure 6, and simplified solid and drift geology maps in Figures 7 and 8 respectively.

The following geological summary is a comparatively brief review of the main observational and interpretative findings of the desk study and the limited field survey undertaken in 1989. It also draws on evidence provided by the original geological survey of the areain 1881 and 1910-13, and subsequent partial revision in the 1970’s. It is intended to beused as an aid to understanding the 1:lO 000 geological base maps. In addition it lists, in context, much of the scientific literature relevant to the area.

The bedrock geology, in outcrop, comprises sedimentary rocks which range in age from high Ordovician (Ashgill) to Permo-Triassic, spanning some 200 million years. An overburden of unconsolidated glacial and post-glacial sediments (Quaternary to Recent) blankets much of the central and eastern partsof the area.

The bedrock sequence has,in general, alow easterly dip; however,in some areas, notably adjacent to the Bala-Bryneglwys fault system, the structure is considerably more complex due to faulting and local folding.

Ordovician

The oldest rocks of the area are of Ashgill (high Ordovician)age and are included in the Cyrn-y-brain Beds (Wedd, Smith and Wills, 1927). They occur onlyin three small areas, eachless than 100 m across, in the bottom of limestone quarries at Minera and in the bed of the adjacent Aber Sychnant. They comprise grey cleavedturbidite siltstones and mudstones. They were deposited in a marine environment some440 million years ago, probably as the distal product of turbidity currents.

Carboniferous

Rocks of Carboniferous age rest unconformably on the Ordovician sediments at Minera. of Rocks Devonian age are absent, as is the case in surrounding areas. The unconformity is an angular relationship, and the Carboniferous rocks, dipping gently to the east and south-east, overlie a previously folded and eroded Ordovician terrain.

Carboniferous Limestone (Dinantian) The first major study of the Carboniferous Limestoneof North Wales was carried out by Morton (1870) who recognised a general regional stratigraphyof Lower Brown, Middle White and Upper Grey limestones. Subsequent workersin this area have tended to substantiateMorton’s classification (e.g. Neaverson, 1946). More recently, Somerville (1979a) has recognised three lithological units, approximately equivalent to thoseof Morton, in the Llangollen-Minera district.

70 The limestones (carbonate sediments)in this report have been describedusing the classification of Dunham (1962). This involves the interpretation of textural features andis ideally suited torapid assessment based on field observations. The Dunham classification has four main categories: (1) grainstone, (2) packstone, (3) wackestone and (4) calcite mudstone. These are defined in the glossary with other less frequently used descriptive categories. In general terms theaverage grain size of the limestones decreases from grainstone through categories 2, 3 and 4. The individual carbonate grains aretypically greater than 0.1mm in size. Classification of the limestones in this way provides direct information relevantto theresource characteristics and engineering properties. Two typesof carbonate grain arerecognised, skeletal and pelloidal.The former were fragmentsof marine fossils and the latterwere formedby sedimentary processes on the sea floor.

There aretwo main outcrops of the Carboniferous Limestone. Thefirst occupies the structurally complex southern side of Hope Mountain and extendswestwards along the northern side of the Bala fault system through Ffrith to Black Wood. In this area the lowest beds seen are within the Cefn Mawr Limestone.The second main outcrop lies to the south-eastof the Minera Fault, on the north-west slopes of Esclusham Mountain, where a complete easterly-dipping sequence unconformably overlies Ordovician sediments. This area is the northern extremity of the limestone outcrop which extends southwards alongEglwyseg Mountain to theVale of Llangollen. Addition- ally there are small areas of Carboniferous Limestone within the Bala fault system between the southern endof Black Wood [SJ 265 5401 and thewestern edge of the study area.

The following sections give brief descriptive and interpretative detailsof the constituent formations:

Loggerheads Limestone Within the study area the Loggerheads Limestone is only present in a small area in and around the Minera limestone quarries [Generalgrid reference SJ 273 5211. It is only about 60 m thick at Minera, althoughit thickens rapidly southwards (outside the study area) to 145 m on Eglwyseg Mountain. It is equivalent to the Eglwyseg Limestone Formation of Somerville (1979a). At Minera thebasal 5 m is referable to the LeeteLimestone (Ty-nant Lime- stone Formationof Somerville, 1979a); these beds have not been delineated separatelythe on map and for the purposesof this report are includedin the Loggerheads Limestone.

The basal 5 m comprises pebblybioclastic limestones and porcellaneouscalcite mudstones; similar beds interbeddedwith pale grey poorly fossiliferous calcite mudstones occupy a further 10 m.The main body of the formation consists of pale grey, cream and white, massive to rubbly bedded, pseudobrecciated, pelloidal and skeletal packstones and subordinate packstone-grainstones.

An important feature of the limestone is the cyclic repetition (‘minor cyclicity’) of variations in lithology (Somerville, 1979a; Somerville and Strank, 1984a, 1984b). Within eachcycle there is an upward increase in grain size with packstones passing up into grainstones. In lower parts of the formation thenext cycle succeeds with or without an initial coarse-grained bioclastic deposit, and then a sharpreversion to finer-grained lithologies. In places, stylolites andsmall carbonate nodules are developed between cycles. Dark grey-brown mottling (patches of0.5-2 cm) is also common in the lower parts of many of the cycles. Towards the topof the formation, thecycle boundaries are made more obvious by the appearance of irregular bedding plane surfaces within which are

71 preserved fossil plant rootlets and roots.In some cases the surfaces possess hollows or pits within which light grey clays are preserved. The topsof these cycles are commonly rich in coarse fossil debris. Laminar carbonate (calcrete) is sometimes observed and iron oxide staining is common. The clays themselves have been interpretedas being potassic bentonites (alteredvolcanic ash) and represent a fossil soil (palaeosol) preserved on an irregular karstic (palaeokarstic) surface with depressions up to 50 cm deep. Each cycle is interpreted as a gradual shallowing of the general marine carbonate platform environment, with eventual emergence of a low topography land surface. The surface was subsequently drowned andfollowed by another phase of gradual shallowing of the sea. George(1974) and Ramsbottom(1973,1977) suggest interpretationsfor thecyclicity. The occurrenceof basal pebbly beds andthe thinning of the formation Mineraat has been attributed to contemporaneous uplift of the Cyrn-y-brain anticline (Somerville,1979a).

Cefn Mawr Limestone This formation is the equivalent of the Trefor Limestone Formation of Somerville (1979a). On the northernside of the Bala fault system the basal part of the formation is faulted out; some75 m of beds are seen. Southof the fault system around Minera the formation, here presentin its entirety, has thinnedto about60 m.

The formation mostly consists of thinly bedded, brown and dark brown, fine-grained pelloidal, skeletal packstones, wackestones andshaly mudstones. Interbedded,mainly towards the base, are thicker bedded, pale, coarser packstone-grainstones. Although the Cefn Mawr Limestone is substantially finer grained overall thanthe Loggerheads Limestone, a similar cyclicity is present. However thekarstic surfaces and bentonitesare rare,suggesting that emergence from thegeneral low energy marine carbonate environmentwas less common.

The base of the formation is marked by an obvious colour changein the rocks, with the brown and dark brown limestones of the Cefn Mawr Limestone contrastingwith the pale grey of the underlying Loggerheads Limestone.

A thick (3-4 m) mudstone (shale) bed is present in the middle part of the formation.This bed, the ‘Main shale’ of Earp (1958), has been mapped on the northern sideof the Bala fault system. It is present to the south buthas not been shown on the map. Acoral-rich bed with abundant colonial and solitary corals occurs approximately 10 m above the ‘Main shale’; it has also been mapped to the northof the fault system.

The upper partsof the formation arevery mudstone-rich (c.50%). White andblack chert nodules, up to10 cmin size, often occur along selected bedding planes,mainly at high levelsin the formation. The formationhas a rich fossil content, andlarge brachiopods are very common.

Minera Formation This formation is about 180 m thick to the northof the Bala fault system, but only about 60 m thick to the south.It resembles the underlying Cefn Mawr Limestone exceptfor the presence of massive and cross-laminated, fine- to medium-grained and occasionally pebbly, calcareous and quartzitic sandstones up to 25 m thick. The bases of the sandstones are sharp. Scattered fossils, trace fossils and carbonaceous material occur in the sandstones. Pebbles in the sandstones are generally of vein quartz. The sandstones,which represent a significant change in

72 sediment provenance, are either marine transgressive sheet-sands or prograding fan-delta systems. They have not been mapped separately southof the Bala fault system.

Millstone Grit Series (Namurian) Halkyn Formation -general The nomenclature and age of the beds between the Carboniferous Limestone and the Productive Coal Measures have been a matterof controversy for many years. To the northof the Bala fault system these beds were subdividedby Wedd and King (1924) into the Cefn-y-Fedw Sandstone (Millstone Grit Series) at the base, overlain by the Holywell Shales and Gwespyr Sandstone which they classified as ‘Lower Coal Measures’. South of the fault the whole of the sequence was termed the Cefn-y-Fedw Sandstone by Morton (1876-9) and Wedd, Smith andWills (1927); these beds include sandstones, conglomerates, cherts and mudstones. The sandstone at the topof the sequencewas known as the Aqueduct Grit.

Correlation of the sequences acrossthe Bala faultsystem was partly achieved by Wood (1937). He showed, by study of the goniatite faunas, that the Holywell Shales in the north were not of Coal Measures age but the equivalentof part of the Cefn-y-Fedw Sandstone to the south, andalso that the Gwespyr Sandstone and Aqueduct Grit were approximate time equivalents since both were overlain by shales containingthe Subcrenatum Marine Band, basal the marker bedof the Westphal- ian Series (Coal Measures).

In the present account, as in the Deeside Report (Campbell and Hains, 1988), the whole of the sequence between the topmost Carboniferous Limestone (Minera Formation) and the of base the Coal Measures (Bettisfield Formation) has beenincluded within one formation, the Halkyn Formation, throughout the study area. Init isage approximately equivalentto theNamurian Series.

Halkyn Formation- north of Bala fault system In this area, as in the Deeside area to the north, a tripartite division into basal sandstone, a mudstone group(‘Holywell Shales’) and the Gwespyr Sandstone is still valid.

The basal sandstone group (estimated to be up to 260 m thick) comprises massively bedded quartzitic sandstones with some subsidiary mudstones and cherts. It occupiesthe northern part of Hope Mountain andmuch of the poorly exposed ground west and southwest of Llanfynydd. The overlying mudstone group (c.200 m thick) is very poorly exposed; it includes grey and black mudstones and shales intercalated with thin quartzose sandstones. Its depositional environment was fluvio-deltaic with periodic marine incursions. The Gwespyr Sandstone (c.60 m thick) is a feldspathic sandstone, in contrast to the quartzitic basal, sandstones and the sandstones of the underlying Minera Formation. As such it is similar to the typical feldspathic sandstones of the overlying Coal Measures.

Halkyn Formation - south of Bala fault system A tripartite division of the formation has also been made in this area; the main difference from the sequence further north is that the basal sandstone group now occupies some 75-90% of the total thickness of the formation, with the mudstonegroup correspondingly reduced in significance. The basal sandstonegroup iswell

73 exposed in quarries along the southern side of the Nant y Ffrith from Bwlchgwyn to Glascoed; elsewhere exposureof all three divisions is generally poor.

The basal sandstone groupis some 260 m thick in the north,thickening southwardsto about360 m within the study area. comprises It massive-bedded fine-to medium-grained quartzitic sandstones, with some thin beds of quartz conglomerate, intercalated with cherts, ‘cherty shales’ and mudstones. The cherts and‘cherty shales’ are completely or partially silicified mudstones and siltstones.

The overlying mudstone group is c.30 m thick in the north, thinning southwards to c.12 m. It consists of grey and black mudstones with thin interbedded quartzitic sandstones.

The Gwespyr Sandstone (Aqueduct Grit)is up to50 m thick in the north,decreasing southwards to c.12 m. It is a feldspathic sandstone varying from fine- to coarse-grained, and locally pebbly especially towards the base. The change in character from the quartzitic sandstonesof the main body of the Halkyn Formation and the underlying Minera Formationto thefeldspathic nature of the Gwespyr Sandstone and the overlying Coal Measure sandstones coincideswith a changein the direction of their derivation; the quartzitic sandstones were derived from the south, whereas the younger feldspathic sandstones were derived from the north-east (Ramsbottom, 1974).

The modeof formation of the cherts and cherty shaleis problematical. The main point at issueis whether the chert formedas a silica gel on the sea floor or originatedby the replacementof existing strata. The evidence here, in and the Deeside areato the north (Campbell and Hains,1988) suggests that replacementis an important factor and thatmuch of the process of silicification occurred soon after deposition of the stratawhile they were still plastic. There is local interbedding of chert with siltstone and mudstone,suggesting that thechertified strata werelikely to have beensiltstones and mudstones prior to their silicification. Palaeontological evidence from the Halkyn area, Deeside, suggests that they were depositedin a low salinity, low energy, possibly lagoonal environment.

Coal Measures (Westphalian) In the British Isles it is common practiceto subdivide the Coal Measuresin terms of the biostratigra- phy (i.e. fossil content) rather than lithostratigraphy (variation of rock type). Thus, elements of the sequence are referred to as Westphalian A,B,C and D. This is a questionable method of depicting stratigraphywithin what is otherwise a lithostratigraphic context, and in this report the termBettisfield Formation (from Bettisfield Colliery, Bagillt, Deeside) has been used as a lithostratigraphic name for the Productive Coal Measures (the ‘Grey Measures’ of Calver and Smith, 1974). Its base is defined at the topof the Gwespyr Sandstone. Thisusually lies a few metres below the Subcrenatum Marine Bandwhich marks the baseof the Westphalian. Its upperlimit is marked by the incoming of the red beds of the Ruabon Marl Formation.

The study area includes the southern partof the ‘Flintshire Coalfield’around Leeswood, and the northern part of the ‘Denbighshire Coalfield’, the two areas separatedby the Bala fault system. There are somedifferences in the sequences within the two areas which are shown on Figure 13. The most recent accounts of the North Wales Coalfields are thoseof Magraw and Calver (1960) and Calver and Smith (1974). The earlieraccounts of Wedd and King (1924) and Wedd,Smith and

74 Wills (1928) provide important detail. The Coal Measures aredivided into four lithostratigraphic formations within the study area; theBettisfield Formation (‘Grey Measures’of Calver and Smith (1974)) at the base is succeeded by the Ruabon Marl Formation, Coed-yr-Allt Formation and Erbistock Formationwhich together make up the ‘Red Measures’. Workablecoal seams are present only in the Bettisfield Formation. Coal Measures occupy the greater part of the study area (Figure 7). In general, exposure is very poor, mainly due to the considerable cover of superficial deposits, and most information derives from boreholes, mine-shafts and undergroundworkings.

Bettisfield Formation (Productive Coal Measures) - general This formation consists of cyclic sedimentary sequences which ideally comprise: basal grey and dark grey mudstones, followed by siltstones; a yellowish brown, feldspathic, cross-bedded sandstone; a seatearth (fossil soil), and coal. The coal is then succeeded by the mudstone at the base of the next cycle. Mudstone is the dominant lithology of the cycles. Some bands of dark grey mudstone, which usually occur imme- diately above the coals, contain marine fossils including stratigraphically diagnostic goniatites. Non-marine bivalves are also used in correlation.

The cyclicity of the sediments demonstrates a systematic patternof subsidence andinfill of the basin within which they accumulated. The overall sedimentary regimewas of fluvio-deltaic sedimentation in shallow brackish water with occasional marine influxes. The sandstoneswere deposited bothby sheet-floods andby migrating channels.The coals represent condensed vegetationwhich accumulated in stagnant swampy conditions, whereas the seatearths which underlie them reflect soils in which vegetation flourished, but without accumulating as a coal.

The Bettisfield Formation is c.510-520 m thick in the ‘Denbighshire Coalfield’ and c.405 m thick in the ‘Flintshire Coalfield’ (Leeswood area), north of the Bala fault system, where the topmost beds are not preserved. The strata of the Bettisfield Formation dip steadily eastwards under the overlying ‘Red Measures’. Most of the more recent deep mining in the Coalfield, from collieries such as Bersham, Gresford and Llay Main, was beneath this cover, often at considerable depth.

A correlation of the principal coal seams within the area is given in Figure 13.

The detailed succession in the Bettisfield Formation will be described in two parts: firstly, the measures upto theHollin coal, in which the sequenceis similar in both coalfields and secondly,the measures above the Hollin coal in which there aresignificant differences between the two areas.

Bettisfield Formation - measures up to the Hollin coal The marine bands commonly present in the lower part of the British Coal Measures are poorly represented in this area. The Subcrenatum Marine Band, marking the base of Westphalian A, has been recorded in both coalfields, though not within the study area. The only proven marine horizon within the study area is the Llay (Vanderbecki) Marine Bandwhich marks thebase of Westphalian B.

The thickness of the beds up to the Hollin coal varies from 240 to 260 m. The principal coal seams, in ascending order, are the Chwarelau, Llwyneinion Half-Yard, Premier, Ruabon Yard, Nant, Nine Foot, Fireclay Group (a variable groupof seams which includes the Firedamp,Half Yard and

75 Stone), Red, Crown, Lower Bench, Main, Black Bed, Quaker, Crank andHollin. The Premier, Nant, Nine Foot and Hollin are commonly split. The thickest of the seams are the Premier (0.66-1.67 m), Main (1.82-4.55 m), Quaker (1.07-2.05 m) and Hollin (0.93-3.00 m). The Nant (up to 3.40 m) and Nine Foot (up to2.16 m) are more variable in thickness. The Main is consistently the thickest seamwithin the Bettisfield Formation. Sandstones are commonly developed above the Premier, Ruabon Yard and Main seams, that above theMain being known as the Main Rock.

Bettisfield Formation-measures above the Hollin coalIn this part of the sequence there are major differences between the two coalfields. In the ‘Denbighshire Coalfield’ several marine horizons have been recorded, but none is known from the Leeswood district. Those in the ‘Denbighshire’ area are, in ascending order: the Powell (Maltby) Marine Band above the Powell seam; the Gardden Lodge (Clown) abovethe Drowsell coal; the Lower Stinking (Haughton) abovethe Lower Stinking seam; the Warras (Aegiranum) above the Warras coal, marking the base of Westphal- ian C, and the Ty Cerrig (Edmondia) above the Upper Stinking coal. Marine Band names in brackets are the standardterminology used in all the British Coalfields.

The thickness of the strata in the ‘Denbighshire Coalfield’ ranges from 230 to 265 m; in the Leeswood area, where the succession is incomplete, 145 m of beds are preserved.

The main coal seams of the ‘Denbighshire Coalfield’are, in ascending order, thePowell, Drowsell, Lower Stinking, John O’Gate, Warras, Cannel, UpperStinking, Gwersyllt Little andthe Bersham Yard Group (avariable groupof up to seven coals, denotedby the Greek letters Gamma to Alpha). The Upper Stinking, Beta and Alpha seams are usually split. The thickest of the seams are the Powell (0.51-1.22 m), Drowsell (0.69-2.29 m), Upper Stinking (1.60-4.00 m), Gamma ( .69- 1.22 m) and Beta(1.19-3.99 m). The Lower Stinking(0-2.00 m), Gwersyllt Little (0-1.47 m), Delta Bench (0.40-1.62 m) and Alpha (0-1.88 m) are more variable in thickness. A thick sandstone, the Cefn Rock (6-45 m), and other sandstone beds,occur between the Gwersyllt Little seam and the Bersham Yard Group.

In contrast there are only four significant seams within the Leeswood area, namely the Powell (0.61-1.35 m), Tryddyn Half Yard, Pontybodkin Divided (1.31-1.80 m) and Pontybodkin Moun- tain. The Pontybodkin Divided is a split seam which, with the Tryddyn Half Yard, is thought to correlate with the John O’Gateof ‘Denbighshire’. A thick sandstone, theHollin Rock (17-35 m), is developedabove the Powell Seam;this is ata lower horizon thanthe Cefn Rock of ‘Denbighshire’.

Ruabon Marl Formation The Ruabon Marl Formation is the lowest of the three formations included within the ‘RedMeasures’ of Calver and Smith (1974). It is between 120 and 160 m thick in the northern part of the study area, thickening southwards to 200 to 225 m. It comprises red, brown, purple and green mudstones,commonly mottled, and thin siltstones and sandstones, with a few thin limestones and some thin grey mudstones and thin coals. The outcrop of the Ruabon Marl Formationis almost completely drift covered, theonly section being in the HafodClay Pit [SJ 310 4541. These sediments were probablylaid down in shallower morebrackish water thanmost of the Productive Coal Measures. Their age is poorly constrained due to a lack of fossils, but the

76 boundary between WestphalianC and D is thought to approximate to the boundary between the Ruabon Marl Formation and theoverlying Coed-yr-Allt Formation (Calver and Smith,1974).

Coed-yr-Allt Formation In contrast to theunderlying Ruabon Marl Formation, theCoed-yr-Allt Formation has itsmaximum thickness of about 200 m in the northeastern partof the study area. It thins westwardsand southwardsto some145-150 m. It comprises grey mudstones, sandy mudstones and sandstones, with parts of the succession showing red and purple bands andmottling. It has a cyclic sequence similar to thatof the Bettisfield Formation, but coals are thin and there area few thin limestone beds.The outcropis almost entirely drift covered, theonly exposures beingin the Alyn Gorge, chiefly between Wilderness Wood [SJ 333 5341 and Bradley[SJ 330 5421.There area number of mine shafts and deep boreholes which penetrate part or all of the formation (Wedd, Smith and Wills, 1928). Its depositional environment shows a partialreturn to thatprevailing during the depositionof the Bettisfield Formation, although therewas no developmentof thick coal seams.

Erbistock Formation The Erbistock Formation is the highest formationwithin the Coal Measures of the study area. As it is overlain unconformably by younger rocks its original thickness is not known; about 520 m of beds are preservedin the northeastern partof the area.

The strata are red and purple mudstoneswith subordinate green banding and mottling, interbedded with coloured andgrey feldspathic sandstones. Some intercalated grey mudstones occur,with thin coal seams, and there are a number of thin limestone beds. The outcrop is almost entirely drift covered, with sections confined to the Alyn Gorge between Wilderness Wood [SJ 333 5341 and Gresford Bridge [SJ 341 5521, and also around King’s Mills [SJ 348 4911. Depositional conditions resemble those obtaining during the depositionof the Ruabon Marl Formation,although the higher proportion of sandstones suggests more active erosionof a nearby land area.

Permo-Triassic

Strata of Permo-Triassic age (between 280 and 210 million years old) overlie the Carboniferous rocks with marked angular unconformity. They occupy the easternof partthe study area (Figure7) and their outcropis completely coveredby thick drift deposits on the edgeof the flat Cheshire Plain. The lithology of the rocks is known from boreholes and from a limited number of sections in adjoining areas (Campbell and Hains,1988; Earp and Taylor, 1986; Poole and Whiteman, 1966).

Two formations are presentwithin the study area; the Kinnerton Sandstone Formation(formerly the Lower Mottled Sandstone) is overlain by the Chester Pebble Beds Formation (formerly the Bunter Pebble Beds).

Kirzrzel-torz Sarzdstorze Forrnatiorz Some 400 m of strata assigned to this formation are thought to be present in the northeastern part of the area. It seems to thin to the south as only about 300 m appear to be presentin the south-east, beneath the Chester Pebble Beds.The Kinnerton Sandstone Formation comprises brownishred and yellow, cross-bedded, fine- to medium-grained sandstones; the beds were depositedby predominantly aeolian processesin a desert environment.

77 Chester Pebble Beds Formation The ChesterPebble Beds Formation is onlypresent in the extreme southeastern corner of the study area, east and north-east of Bangor-is-y-coed. About 140 m of beds are presentwithin the area. They comprisereddish brown, cross-bedded, medium- to coarse-grained sandstones, usually with abundant rounded pebbles; the pebbles, mainly of quartzite, occur scattered throughout the rock, or in lenses. They were laid down by northward flowing streams in the lower reachesof a continental drainage complex.

Structure

The most significant structural feature is the ENE trending Bala faultsystem which traverses the northern part of the Carboniferous outcrop (Figure 7). It has a long and complexhistory and may have been initiated in Precambrian times (a minimum of 600 million years ago) (Fitches and Campbell, 1987). Its most obvious effect in north-east Wales is the sinistral displacement of the main Carboniferous Limestone (Dinantian) outcrop by some 7 km;in detail, however, it is more complex and George(1974) has postulated that the decreasein thickness of the Dinantian sequence from about900 m to thenorth of the fault system to some 175 m on the southern sideis due either to growth faultingor some20 km ofsinistral displacement. Additionally there are some differences in the Halkyn Formation, and considerable differences within the upper part of the Bettisfield Formation (Figure 13), either sideof the fault system which may be relatedto it. Thefault does not affect the Permo-Triassic rocks, althoughit can be tracedby geophysical methods for some distance under the unconformable Permo-Triassic cover.

On the southernside of the fault system the Carboniferous rocks have a consistent easterly dip (see Thematic Map No.1, horizontal section); they are unconformably overlain by the Permo-Triassic rocks which have a general, but less consistent, easterly dip.The Carboniferous rocks are affected by a number of major NNW trending normal faults (e.g. the Wrexham and Gresford faults)with subsidiary conjugate E-W faults. Towards the west of the area the trend becomes NW (e.g the northern part of the Minera Fault), and most of the mineralised faults within the Carboniferous Limestone have this trend.The major faults swing to N or NNE trending towards the southof the study area. There arevery few folds, and those areonly of minor significance.

The area to the northof the Bala fault system is structurally more complex. The major faults still have a NNW trend, and recent exploration for opencastcoal sites has shownthat there aremany minor faultsof similar trend. The Carboniferous Limestone Halkynand Formationon the southern part of Hope Mountain are folded into a broad NE trending anticline, the southern part of the so-called ‘Horseshoe Anticline’of Wedd and King (1924). This anticlineis complicated by numerous anastomosing faults,mainly with a northerlyor northwesterly trend. Itis not known if this mode of faulting extends intothe poorly exposed ground surroundingHope Mountain to the east, north and west.

The dominant N-S (or NNW-SSE) and conjugate E-W normal faults were probably generated during the formation of the extensional basin in which the Carboniferous rocks were deposited. There may have been further movement along many of the faults during the end-Carboniferous (Hercynian) earth-movements. Thereis geophysical evidence that someof the faulting penetrates

78 the overlying Permo-Triassic rocks,and it is possible that this may represent a reactivationof earlier faulting within the underlying Carboniferous sediments. Theextensive drift cover has prevented any detailed studyof structures within the Permo-Triassic rocks.

Mineralisation

The locally intenselead, zinc andcopper mineralisation which occurs in theCarboniferous Limestone southof the Bala faultsystem was strongly influenced, and probably controlled,by the NW-SE faults. The mineralisation occurs mainly in discontinuous veins (?solution infils) along faults and joints,but it also occurs in steep pipes (probably infilled solution pipes). The chief ore is galena (lead sulphide), with sphalerite (zinc sulphide) and minor amounts of chalcopyrite (copperhron sulphide). The main associated vein (gangue) mineral is calcite, although quartz is locally dominant at Minera. There are small amounts of zinc and lead carbonates. At Minera, galena alone is present at thenorth-west end of the mineralised area; towards the south-east blende becomes common and in the extreme south-east is more abundant than galena. This suggests a broad zonal arrangementof the primary minerals (Earp,1958).

North of the study area, in the southern partof the Deeside area (Campbell and Hains,1988), there is little mineralisation in the Carboniferous Limestone above thelevel of the ‘Main shale’. Similar constraints appearto apply at Park Mines, southof Minera. However, at Mineraitself mineralisation extends throughout the Carboniferous Limestone and into the basal part of the overlying sandstones of the Halkyn Formation. It seemslikely that thelarge throwof the faults and themany and varied enclosed lensesof strata have tended tomask the effects of shale bedsin controlling ore distribution within the limestones.

The nature and stratigraphicdistribution of the ores suggest that, in the main, they were precipi- tated in solution voids within the limestone and particularly along pre-existing faults and joints. The stratigraphic restrictions probably reflect the optimum conditions for precipitation of the mineralising fluids (e.gpH).

Quaternary

The areawas affected by several periodsof glaciation duringthe Quaternary, butonly evidence of the last glaciation (Devensian) has been preserved. Glaciation affected a post-Triassic erosion surface and accentuated existing valleys. In addition, new dissections were caused by subglacial and periglacial processes. Ice-sheets from two directions impinged on the area, one from the direction of north and central Wales (the‘Welsh’ ice-sheet) and the other from theIrish Sea to the north (‘Irish Sea’ ice-sheet). Northof Caergwrle these ice-sheets mergedin the vicinity of the River Alyn, and southwardsto the west of Wrexham (Ball, 1982; Dunkley, 1981; Thomas, 1985, 1989). Extensive basal tills (boulder clays) were deposited. These tills, and laterglacial and fluvioglacial deposits, cover mostof the central and eastern partsof the study area, and areoften of considerable thickness, up toa maximum of about 100 m (see Map No.4 for drift isopachytes).

79 The two different ice-sheet sourcesare reflected in the different clast compositions of the two till deposits. The ‘Welsh’ ice-sheet till comprises grey clays with clasts mainly of Lower Palaeozoic cleaved siltstones and sandstones, vein quartz and acidic volcanic rocks. The ‘Irish Sea’ ice-sheet till comprises reddish brown clays with abundant clasts of Carboniferous Limestone, mudstone, sandstone, chert and coal of local derivation, in addition to Triassic sediments and granitic, volcanic, metamorphic andvein quartz clasts of more distant provenance.

During the retreatstage of the ice-sheets, meltwater flowed southwards betweenthe retreatingice margins and laid down a complex series of fluvioglacial and lacustrine deposits in the Wrexham district. There have been a numberof conflicting interpretations of the modeof deposition of these sediments (Peake,1961; Poole and Whiteman,1961; Wilson, Mathers and Cannell,1982; Thomas, 1985, 1989) but it is clear that the extensive sand and gravel deposits around Wrexham (the ‘Wrexham Delta-terrace,of Wedd, Smith and Wills, 1928) were formedas a complex outwash fan deposited, at least partly,in or on themargins of a proglacial lake (Bangor Lakeof Thomas, 1989). Lacustrine silts and clays laid down in this lake, and possibly in other transient lakes,occur to the east and north-east of Wrexham. Minor ice-sheet readvance occurred, resulting in the local interdigitation of till and fluvioglacial sand and gravel.

The retreating ice-sheets left patches of dead ice and the melting of these produced kettle holes (depressions) in the sand and gravel deposits. These were sites of small lakes and ponds and laminated clay and peat accumulatedwithin them. In the periglacial freeze-thaw conditions of the immediate post- glacial era, glacial deposits suffered downslope mass-movement by solifluction processes producing head deposits. Landslip also occurred at this time, both in bedrock and superficial deposits, and it persists to the present day. Someof the major pre-glacial valleys, and particularly parts of the Alyn and Dee valleys, were largely infilled by glacial deposits. Flood conditions continue to infill the valleys with alluvium.

80 ANNEX B DATA SOURCES USED INTHE REPORT

British Geological Survey(BGS) archival maps and data

The primary surveyof the northern partof the study areawas carried out, at the1: 10 560scale, by Strahan andDe Rance (1879-1885). The remainder of the areawas first surveyed, and the northern part partially revised,by King, Lamplugh, MacAlister, Thomas, Wedd Wills and in 1910-1913. The eastern marginof the areawas revised by Poole in 1956. The field-slips relating to all these surveys are held at the Aberystwyth Office of BGS. Descriptive memoirs for all these surveys were produced (Strahan, 1890; Wedd and King, 1924; Wedd, Smith and Wills, 1927, 1928; Poole and Whiteman, 1966; Earp and Taylor 1986), and maps published 1:63at 360 scale. In addition, maps of the coalfield areas were published at 1 :10 560 scale as a result of the 1910-1913 survey. The metalliferous deposits of the area weredescribed in a Mineral Resources Memoir (Smith,1921). Further updatingof part of the areawas undertaken by Lowe, Reedman and Tappinin 1977-1980, and the 1:lO 560 sheet SJ 24 NE was published in 1982. The sand and gravel reserves of the area weredescribed in twoMineral Assessment Reports(Dunkley, 1981; Ball, 1982). Extensive non-confidential and confidential borehole records were held prior to the present project at the Aberystwyth Office of BGS, as were some geotechnical reports and plans and cross-sections relating to metalliferous mining.

Field Survey

Information was obtained from a detailedfield survey of the Hope Mountain areaby Drs Davies and Wilson in 1989, and from a photo-interpretationwith limited field checking forthe remainder of the study area. The Hope Mountain area had not beensurveyed since 1880, and the complex stratigraphy and structure of the rocks required a complete re-interpretation. The observations made during this survey were recorded directlyon to 1: 10 000 field-slips. Standard BGS mapping techniques wereused during thefield survey. These include systematic examinationof all exposures of rock, with detailed descriptions and measured sections recorded where appropriate. Most topographic features greater than approximately 1 metrein elevation weresystematically recorded to assist in interpretation in areas of limited rock exposures. Somefossil identifications were carried out by Dr N J Riley of the Keyworth Office of BGS to help elucidate the stratigraphyof the area. In other areas, black and white air photographs (approximate scale 1:lO 000) were examined, mainly to determine areasof made ground, backfill and landslip. These areas were then checked in the field. The photographs were examined at theClwyd County Council offices in Mold where they hold a complete stereoscopic coverof the study area dating from1984-5.

British Coal Databases

A number of databases areheld by British Coal. Borehole data abandonment plans and other data obtained from the Opencast Executivewas used to derive coal seam crops andother information but the primary data remains confidential.A copy of the British Coal Shaft Register was obtained; from this register, geological fieldslips and other sources, some 1350 shafts and adits have been located with varying degrees of accuracy. Seam plan information was collated from the Coal

81 Commission plans, and other plans held by British Coal, Staffordshire House, Berry Hill Road, Stoke-on-Trent ST4 2NH. A synthesis of this seam plan data is held on maps at 1:10 560 scale at the Aberystwyth Office of BGS, andin a computer database at Keyworth.

Site Investigation Data

Site investigation information for civil engineering projects was obtained from many bodies including the Welsh Office,Clwyd County Council, Alyn and DeesideDistrict Council, Wrexham Maelor Borough Council, the Welsh Development Agency and a numberof geotechnical consultants. This information comprises borehole data and trial pits, with, in 575 cases, details of engineering properties, and in some instances full geotechnical and other reports. Most of the boreholes were drilled by shell and auger rigs and are less than 25 m in depth, and many are restricted to the unconsolidated deposits. The distributionof borehole sites is very uneven, with particular concentrationsin industrial estates and along major road improvement schemes. In total, including British Coal borehole data, records from approximately 4200 non-confidential and confidential boreholes were considered during the project. Detailsof all the boreholes,including summaries of the geological data, have been entered into a computer database (Annex D). Geotechnical data has been entered into a separate, but compatible, database. These databases are held at the BGS Headquarters at Keyworth. The papercopies of the borehole logs are held at the Aberystwyth Office of BGS where the non-confidentiallogs can be consulted.

Hydrogeological Data

The National Rivers Authority and the Welsh Water Authority provided information on water wells and boreholes, water abstraction and quality, chemical analysesof water and onlandfill sites.

Scientific Publications

Various publications relevant to the area are available in the scientific literature. These deal particularly with aspects of the stratigraphyof the Carboniferous Limestone sequence and with the nature and interpretation of superficial deposits within the area. They are referred to where appropriate within the report and full details are given in the reference list. The references are available in the libraries of the British Geological Survey at Aberystwyth and Keyworth. Other useful reference centres are: the London Information Office,British Geological Survey, Geological Museum,Exhibition Road, London SW7 2DE; the Department of Geology, Jane Herdman Laboratories, University of Liverpool, PO Box 47, Liverpool; and forhistorical information with reference to mining and quarrying, Clwydthe Records Office,Clwyd County Council, Hawarden.

Using all the sources listed above, geological maps at the scale of 1 10 000 were prepared. Each rnap delineates both lithological and structural variations in the bedrock geology and the nature and distribution of superficial deposits (drift). In addition the sites of selected shafts, adits and non-confidential boreholes were plotted. Each base rnap has a generalised vertical section with general descriptions of the constituent geological formations. The geological linework has been digitised and other information entered into a computer databaseso that the 1:10 000 geological

82 maps can be computer generated as required. This database, together with those containing information on mine plans, boreholes (including geotechnical data), shafts, adits and other point data such as the position of quarries, pits and kettleholes, has been entered into a Geographic Information System (GIs). A digital terrain model has been produced and also entered into the GIs. Selected elementsof these various databases have been combinedwithin the GIS to generate the thematicmaps.

83 ANNEX C THE GEOTECHNICAL DATABASE

The classification of the geological formations into groupsor units of similar engineering properties was carried outusing geotechnical data extracted from site investigation reports on sites located in, or within 2 km of, the study area. Datawas abstracted, coded ontoa specially designed form and keyed into a computer database using an IBM compatible microcomputer and commercially available software. The geotechnical database is maintained as a separate entity. However,all the boreholes for which data areavailable are also held within the main borehole database, and each borehole has the same accession number in each data set. Geotechnical data were obtained for 4979 individual samples from575 boreholes.

The results of the following geotechnical tests and measurements (laboratory and in situ) were abstracted and entered into the database: 1 Standardpenetration density.6 Dry 11 Compaction. test (SPT). 7 Specific gravity. 12 Californiabearingratio M oisture content. (CBR). content. 2 Moisture 8 Particle size analysis 3 Liquid limit. (PSA) . 13 pH. (acidity/alkalinity)

4 Plastic limit. 9 Triaxial compression 14 Sulphatecontent (of soil (drained and undrained). and groundwater). 5 Bulkdensity. 10 Consolidation. 15quality designationRock (RQD) .

A brief description of the tests and their applications is given below. The following should be consulted for further information: British Standard1377, (1975) and BritishStandard.5930, (1981).

The standardpenetration test (SPT) is a dynamic test carriedout atintervals during the drilling of a borehole, widely used to give an indication of the relative density of granular soils (very loose to very dense), and theconsistency of cohesive soils (very softto hard). Correlationshave also been made between SPT and the bearingcapacity of a soil.

The moisture contentof a soil sample is defined as the ratio of the weight of water in the sampleto the weight of solids, normally expressed as a percentage.It is a basic soil property and influences soil behaviour with regard to compaction, plasticity, consolidation and shear strength characteristics. As moisture is removed from a fine-grained soil it passes through a series of states, that is liquid, plastic, semi-solid and solid. The moisture contents ofa soil at the points where it passes from one stateto the next are known as ‘consistency limits’.

The plastic limit (PL) is the minimum moisture content atwhich the soil can be rolled into athread 3 mm diameter without breaking up.

84 The liquid limit(LL) is the minimum moisture content whichat the soil flowsunder its own weight. The range of moisture content over which the soil is plastic is known as the plasticity index (PI), such that PI = LL - PL.

The factors controlling the behaviour of the soil with regard to consistency are: the natureof the clay minerals present, theirrelative proportions, and the amount and proportionsof silt, fine clay and organic material.A soil may be classified in terms of its plastic behaviour by plotting plasticity index against liquid limit on a standardplasticity (or Casagrande) chart. The consistency chartsalso give an indication of soil strength andcompressibility.

Density of a soil, thatis the mass per unit volume, may be measured in various ways. The total or bulk density is the mass of the entire soil element (solids + water) divided by the volume of the entire element. The dry density is the mass of dry solids divided by the volume of the entire soil element. Soil density measurements may be used to assess various earth loads such as soil mass, overburden pressure, surcharge pressure and earth pressure on retainingwalls.

The specificgravity of a soil is the ratioof the weight of dry solids to theweight of an equal volume of water (ie the weightof water displacedby the solids). It is, therefore,dimensionless a parameter. Specific gravity is a basic soil property and represents an average for the particles of different minerals present in a soil sample. The parameteris used to enable calculation of other basic soil properties.

Particle size distribution is used for classifying soil in engineering terms. Particlesize distribution curves will give an indication of soil behaviour with regard to permeability, susceptibility to frost heave or liquefaction and will give some indication of strength properties.Particle size analysis does not, however, indicate structure. For example a sandyclay and a laminated sand and clay which may behave verydifferently in situ, may show similar particle size distribution in bulk test sample.

The triaxial compression test is the most widely used test for determining the shear strength of cohesive soils, and a numberof different methodsmay be used depending on theapplication of the results. The test may be carried out with the sample either drained or undrained and the typeof test will depend upon the site conditions and typeof engineering works being undertaken.

Graphical interpretation of the results enables the shear strength of the soil to be determined in terms of its cohesive and frictional components: undrained (apparent) cohesion,C, and angle of shearing resistance (internal friction), U.

If saturated cohesive soil is subjected to an increasein loading the pressureof the waterin the pore spaces will increase by the same amountas the applied stress. The waterwill therefore tend toflow towards areasof lower pressure at a rate controlledby the soil permeability. The removalof water causes a decreasein volume of the soil, a process knownas consolidation. The coefficient of volume compressibility Mv (m'/MN), is a measure of the amount of volume decrease that will take place 7 for agiven increase in stress. Thecoefficient of consolidation, Cv (m-/year), is a measureof the rate at which the volume changewill take place for agiven increase in stress. Theresults of consolidation

85 tests give Mv and Cv at a numberof increasing loads. To enable this rangeof values to be used in the database theranges are converted toclasses using the tableswhich have been taken fromHead (1982) and Lambeand Whitman(1979). The assignment to aclass is mainly based onthe mid-range values obtained during progressive testing.

Coefficient of volume compressibility (Mv) Coefficient of consolidation (Cv) m2/MN Class Description of m2 lyr Class compressibility H.5 5 Very High 100 5

Consolidation test resultsare important for foundationdesign and calculating the likely settlements that will take place during and after construction.The test results also enablethe planning of phased construction stages toallow full consolidation settlement (dissipationof pore pressure) to take place prior to successive load stages.

The compaction test determines the‘optimum’ moisture contentat which a soil may be compacted to its maximum dry density. The dry density of the compacted soil is plotted against its moisture content and the moisture content atwhich maximum compacted density may be achieved is read from the curve. Theresults of the compaction test are used to determine the optimummoisture conditions at which to place a given soil as general or embankmentfill.

The California bearing ratio test is an empirical test carried out in the laboratory, or in the field, which compares the resistance of a soil to penetration by a standard plunger to the resistance to penetration shown by a standard crushed stone:

CBR = (Measured force)I (‘Standard Force’)

The CBR value of recompacted soil is very sensitive to variations in moisture content and dry density. The results of the CBRtest are used to assess the suitability of soils for use as base, sub-base and sub-grade in road construction.

The pH of soil or groundwater is important when designing concrete structures below ground surface. Ordinary Portland cement is not recommended in situations with a pH below 6, high alumina cement can be used down to pH 4 and supersulphated cement has been used to pH 3.5. Acidic groundwaters can also cause corrosion in buried metal work.

It is also important that thesulphate contentof groundwater andsoil is known, as ordinary Portland cement deteriorates in the presence of sulphate. Building Research Establishment Digest No. 250 (1981) discusses the factors responsible for sulphate attack on concrete below ground level, and recommends what can be done, by suitable selection of cement type and concrete quality,resist to

86 attack by naturally occurring sulphates.Part of the appropriate tableis reproduced below, but the document should be consulted for further information.

Allocation to sulphate class in concentrations of sulphate expressed as so3

Concentrations of sulphates expressed as so3

in soil in groundwater Class Total so3 (YO) SO3 in 2:l water: 811 soil extract g/l 1 <0.2 <1.0 <0.3 2 0.2 to 0.2 2 0.5 1.0 to 1.9 0.3 to 1.2 3 0.5 to 1.0 1.9 to3.1 1.2 to2.5 4 2.0 to 1.0 3.1 to 5.6 2.5 to5.0 5 >2 >5.6 >5 .O

Rock quality designation (RQD) was introduced togive an indication of rock quality in relation to the degreeof fracturing fromdrill cores. Itis defined as a sum of the coresticks in excess of 100 mm in length expressed as a percentage of the total length of core drilled. RQD has been used with uniaxial compressive strength to give an indication of excavatibility, and as one input for the classification of rock massesto assist in the design of tunnel supportsystems. Forfurther information see Deere(1964).

87 ANNEX D THE BOREHOLE DATABASE

BGS holds an extensive collection of paper recordswhich describe the strata encounteredduring the sinking of boreholes, mineshafts and trial pits. These records represent a majorof sourcefactual data for future geological investigations and interpretations within the study area. The paper borehole recordis considered to be the authoritative primary record and there has been no attempt to replicate it on the computer. A computerised database for the project wasarea seen as away of utilising the dataheld in the paperarchive to maximum effect by allowing rapid andflexible retrieval of a wide range of information.

All borehole and shaft recordsin the project area have been coded following a format which has been developed within BGS. This work has resulted in the establishment of a simple system by which borehole information can be assigned to two tables of data, on the basis of the following criteria:

All data which describes a feature of the borehole itself, for example, its name, its location by National grid reference, the dateit was drilled, height above (or below) sealevel.

All information relating to samples taken from point depthsbelow the surface for that borehole.

A complete list of fields in each of these tables within the borehole databaseis given below.

The datais currently held in an Oracle relational database managementsystem running on aVAX 8560 computer at BGS Keyworth.It can be interrogated by menu or specifically by using a query language (SQL). It may be used as an index to available recordsor for detailed geological analysis. Retrieved data may be manipulated and displayed using a number of statistical and graphic application packages and it is possible to automatically produce graphic vertical sections (Figure lo), scatter plots, contour maps and three-dimensional diagrams. Several elements within the thematic maps were producedby retrievals from the database.

List of fields in the BGS Keyworth borehole database Index table Lithological table

OS 1:10 000 sheet number” Reliability OS 1: 10 000 sheet number* Accession number and suffix” Start point Accession number andsuffix* National grid reference andac- Inclination Depth curacy Drilling date Lithology Borehole name Geologist Base of bed Comments Drilling method Reliability Surface level and accuracy Borehole diameter Workings Confidentiality Consulting engineer Comments Purpose Drilling contractor Originator Water strike * “the primary key index”

88 ANNEX E GLOSSARY

Alluvium Detrital material transported by a river and deposited alongits floodplain

Anticline Rocks folded in the form of an arch

Aquifer A body of rock that containssufficient saturated permeable material to conductGround- water and toyield economically significant quantities of groundwater towells, boreholes andsprings

Argillaceous A deposit containing an appreciable amount of clay

Artesian head The hydrostatic headof an artesian Aquifer, or of the water in the aquifer

Artesian pressure Hydrostatic pressureof artesian water, or height above landsurface of a column of water that would be supportedby the pressure

Basin A depression in which sediments accumulate

Bedding The arrangement of a sedimentary rock in beds or layers of varying thickness and character

Bedrock Geological strata at surface and below Superficial deposits

Bioclastic A sediment comprising fragments of Skeletal material

Bivalve A (fossil) mollusc with two shells which are symmetrical with respect to each other

Brachiopod A marine animal (fossil) with two symmetrical shells

Buried channel An old channel concealed by Drift deposits

Calcareous A deposit that contains calcium carbonate

Calcite A common rock-forming mineral, CaC03

Calcite mudstone A Limestone similar to a Wackestone, but in which the carbonategrain content must be less than 10% of the whole rock

Calcspar Coarsely cystalline calcite, usually in veins

Carbonaceous A deposit that contains organic matter

Cementstone A fine-grained Calcareous siltstone or Limestone

89 Chert A rock comprising very fine-grained quartz

Clast An individual grain or rock fragment

Cleaved Pertaining to a rock fabric imposed by compression

Cohesive A sticky Soil like clayey silt

Conglomerate A coarse-grained sedimentary rock with average clast size greater than 4 mm

Correlation The process by which Stratigraphic units in two or more areas are demonstrated to be temporally equivalent

Crinoid A marine organism (fossil) with a structure comprising discs of calcite

Cross-bedding Structure insedimentary rocks comprising intersecting Bedding planes

Cyclicity A repeated sequence of beds or rock units

Debris flow A depositional mechanism akin to a mud slide

Dip The angle of inclination of a surface (e.g. Bedding) in relation to the horizontal

Disharmonic Relating to folds which differ in form with respect to one'another

Distal Distantfrom

Drift A general term for all superficial unconsolidated rock debris of Quaternary age distin- guished from solid bedrock

Facies change A change in the internal characteristicsof a rock unit

Fault A surface or zoneof rock fracture along which displacement has occurred;movement may either bevertical (normal or reversed), lateral(strike-slip) or a combination of the two

Feldspathic A deposit that contains the mineral feldspar in significant quantity

Field-slip The base map onwhich geological observations and comments arerecorded

Fireclay a Seatearth comprising quartzose clay, and capable of withstanding high temperature without deforming

Formation The fundamental unit of subdivision of a rock sequence which is unified in internal characteristics anddiffers with respect to adjacent formations

90 Geotechnical The application of scientific methods and engineering principlesto theacquisition, interpretation, and use of knowledge of materials of the Earth’s crust to the solution of civil engineering problems

Goniatite A fossil mollusc with a shell of spiral form

Graben A sequence of strata between two Faults, that is lowered relative to adjacent strata

Grainstone A Limestone where the individual grains of carbonate arein direct contact (grain-supported) andfine-grained carbonate mudis minimal

Gravel An unconsolidated accumulation of rounded grains of average grain size greater than 4 mm

Groundwater Water contained in the soil or rock below the water-table

Hardness A property of water causing formation of an insoluble residue

Horizontal section An interpretive diagram displaying the structure and sequence of strata at depth

Horst A sequence of strata between two Faults, that is raised relative to adjacent strata

Igneous Rock which was originally molten

Induration A process by which a soft sediment becomes a hard rock (hence Indurated)

Isopachyte A line joining points of equal bedor deposit thickness

Karstic Relating to irregular topography of a limestone surface by its dissolution

Kettle hole A steep-sided depression in glacial deposits, usually containing clays and peat laid down in a lake or swamp

Lacustrine Pertaining to a lake

Laminated A very fine type of Bedding less than 1 mm thick

Limestone A sedimentary rock mainly comprising calcium carbonate

Lithological Pertaining to rocks

Lithostratigraphical Pertaining to the classification of rocks on the basis of their physical charac- ters

91 Mature (ofSandstone) Pertaining to a predominance of well rounded and spherical grains

Member A distinctive unit of strata within a Formation

Metamorphic Pertaining to rocks which have been affected by high temperature and/or pressure resulting in changes in their mineral constituents

Mudstone A sedimentary rock comprising very fine-grained particles

Nodule (phosphatic, ironstone, carbonate) A small body of mineral or mineral aggregate, often rounded or ellipsoidal in profile

Overconsolidated Clay that still retains some of the imposed stress from a previous greater overburden

Packstone A Limestone which is generally grain-supported (see Grainstone)but with significant fine-grained carbonate mud matrix,in some instances resultingin separation of the carbonategrains

Palaeontological Pertaining to thestudy of fossil plants and animals

Particle size distribution The percentage of particles in each size fraction of a sample of soil, sediment or rock; theresult of particle size analyses

Pelloid (Peloid) A grain formed of fine-grained Calcareous material irrespective of size or origin

Periglacial Said of the processes occurring inthe region adjacent to glaciers and ice sheets

Peritidal A sedimentary environment within or near to the tidal range

Permeability The property orcapacity of a rock, sediment orsoil for transmitting a fluid pH The measure of the acidity or alkalinity of a solution

Pipe A cavity, usually in calcareous rocks, commonly filledwith clay, sand or gravel

Platform (as in carbonate) Pertaining to a very gently sloping sea bed, with little topography

Porcellanous Pertaining to a very fine-grained calcareous sediment resemblingunglazed porcelain

Porosity The property of a rock, soil or other material containing interstices; it is commonly expressed as a percentageof the bulk volume of material occupiedby interstices, whether isolated or connected

Proglacial The environment in front of a glacial ice sheet

92 Prograding Relating tosediments laid down as a deposit which builds out in front of the source of the sediments

Pseudobrecciated Pertaining to a texturein limestoneresembling a fragmental rock,but is produced by alteration of the limestone

Quartzitic A deposit mainly comprising quartz

Quartzose A deposit containing quartz

Rock(Engineering) A naturally found material with a uniaxial compressive strength overa certain minimum value (usually taken as 1 MN/m2), and composed of mineral grains

Rockhead The interface between unconsolidated Superficial deposits and Bedrock (and usually taken as thebase of the weathering profile of the bedrock)

Running Mass flow of non-cohesive Sand due tohigh water content

Sand(stone) A sediment (consolidated) comprising grains between0.063 and 4 mm in averagesize

Seatearth A unit, generally of clay, that underlies a coal and represents thefossil soil on which the vegetation grew

Shale A Mudstone with bedding parallel fissility

Sheet-sand A unit of Sandstone of wide areal extent and limited thickness variation

Silt(stone) A sediment (consolidated) comprising grains between 0.063 and 0.004 mm in average size

Skeletal Carbonate fragments of organic origin

Slumped Pertaining to sediment disrupted on the seafloor due to gravitational collapse

Soil (Engineering) All material formed from aggregatesof rock particles which can be separated by gentle mechanical means and excavated without blasting

Solid Bedrock geology (excluding Drift deposits)

Solifluction A process involving the slow downslope movement of superficial material as a result of the alternatefreezing and thawing of the containedwater

Standard The fair copy of the geological map (scale 1:10 000or 10 560)

93 Stratigraphic Pertaining to thestudy and classification of the sequenceof rock strata in the Earth’s crust

Strike The orientation of a horizontal line drawnon an inclined surface (e.g. Bedding)

Stylolites Irregular surfaces within limestone due to dissolution, and with insoluble residue preserved along the contact

Subartesian Said of confined Groundwater that is under sufficient pressure to rise above the water-table, but not to the land surface

Superficial deposits Unconsolidated glacial and postglacial sediments

Swallow holes Closed depressions or dolines into which all or part of a stream disappears underground

Syncline Rocks folded in the formof an inverted arch

Tectonic Pertaining to forces which deform or disrupt strata

Trace fossilA sedimentary structurecomprising the fossilised burrow or track of a marine organism

Transmissibility In an Aquifer, the rateof flow of water through each vertical stripof the aquifer having a height equalto thethickness of the aquifer and under aunit hydraulic gradient

Turbidite The sediments deposited from aTurbidity current

Turbidity current A turbulent mixtureof sediment and water which flows on the seafloor under the influence of gravity

Unconformity A break in the sedimentary sequence with strata of a particular age absent and possibly an angular relationship betweenthe strataof different ages

Vein A mineral infilling of a fault or fracture in a rock, andusually sheet-like

Vertical Section A diagrammatic representation of vertical variations in a sequenceof strata

Vibroreplacement A technique for improving the load-bearing characteristics of soft, cohesive Soils. Depending upon its undrained shear strength (see Annex C), the soil is either displaced radially by use of a vibratingprobe, orremoved to thesurface by washing out with avibrating probe with water jetting. Thehole is infilled, in stages, with well-graded 75-100 mm angular stone which is compacted within the vibrator. The stone columnsprovide vertical supportfor overlying structures, and also function as vertical drains.

94 Wackestone A Limestone where the fine-grained carbonate mud matrix is predominant and individual carbonate grains(which must comprise more than10% of the rock) are rarely in contact with one another (i.e. thegrains are matrix-supported)

Well sorted Pertaining to a sedimentary rock comprising particles all approximately of the same size

Zonal Pertaining to a subdivision (zone) of strata based on its fossil content

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95 I